Acts 8:14-17 - the people of Samaria were baptized in Christ, but did not receive the fullness of the Spirit until they were confirmed by the elders. Confirmation is a sacrament that Jesus Christ instituted within His Catholic Church to further strengthen those who have reached adulthood.
Acts 19:5-6 - the people of Ephesus were baptized in Christ, but Paul laid hands on them to seal them with the Holy Spirit. This sealing refers to the sacrament of confirmation.
Eph. 1:13 - Paul writes that the baptized Ephesians were sealed with the promised Holy Spirit, in reference to confirmation.
Eph. 4:30 - Paul says the Ephesians were sealed in the Holy Spirit of God, in reference to the sealing of confirmation.
Heb. 6:2 - Paul gives instruction to the Hebrews about the laying on of hands, in reference to confirmation, not ordination. The early Church laid hands upon the confirmand to administer the sacrament of confirmation.
Heb. 6:2 - this verse also refers to the cycle of life and its relationship to the sacraments - baptism, confirmation, death and judgment - which apply to all people.
John 6:27 - Jesus says the Father has set His seal on Him. As the Father sets His seal on Jesus, so Jesus sets His seal on us on the sacrament of baptism, and later, in the sacrament of confirmation.
Rev. 9:4 - the locusts could not harm those with the seal of God upon their foreheads. See also Rev. 14:1 and 22:4.
Tradition / Church Fathers
"And about your laughing at me and calling me "Christian," you know not what you are saying. First, because that which is anointed is sweet and serviceable, and far from contemptible. For what ship can be serviceable and seaworthy, unless it be first caulked [anointed]? Or what castle or house is beautiful and serviceable when it has not been anointed? And what man, when he enters into this life or into the gymnasium, is not anointed with oil? And what work has either ornament or beauty unless it be anointed and burnished? Then the air and all that is under heaven is in a certain sort anointed by light and spirit; and are you unwilling to be anointed with the oil of God? Wherefore we are called Christians on this account, because we are anointed with the oil of God." Theophilus of Antioch, To Autolycus, I:12 (A.D. 181) .
"'And she said to her maids, Bring me oil.' For faith and love prepare oil and unguents to those who are washed. But what were these unguents, but the commandments of the holy Word? And what was the oil, but the power of the Holy Spirit, with which believers are anointed as with ointment after the layer of washing? All these things were figuratively represented in the blessed Susannah, for our sakes, that we who now believe on God might not regard the things that are done now in the Church as strange, but believe them all to have been set forth in figure by the patriarchs of old, as the apostle also says: 'Now these things happened unto them for ensamples: and they were written for our instruction, on whom the ends of the world are come.'" Hippolytus of Rome, Commentary on Daniel, 6;18 (A.D. 204) .
"After this, when we have issued from the font, we are thoroughly anointed with a blessed unction,--a practice derived from the old discipline, wherein on entering the priesthood, then were wont to be anointed with oil from a horn, ever since Aaron was anointed by Moses. Whence Aaron is called "Christ,' from the 'chrism, 'which is 'the unction;' which, when made spiritual, furnished an appropriate name to the Lord, because He was 'anointed' with the Spirit by God the Father; as written in the Acts: 'For truly they were gathered together in this city against Thy Holy Son whom Thou hast anointed.' Thus, too, in our case, the unction runs cornally, (on the body,) but profits spiritually; in the same way as the act of baptism itself too is carnal, in that we are plunged in water, but the effect spiritual, in that we are freed from sins." Tertullian, On Baptism, 7 (A.D. 206) .
"But Satan, who entered and dwelt in him for a long time, became the occasion of his believing. Being delivered by the exorcists, he fell into a severe sickness; and as he seemed about to die, he received baptism by affusion, on the bed where he lay; if indeed we can say that such a one did receive it. And when he was healed of his sickness he did not receive the other things which it is necessary to have according to the canon of the Church, even the being sealed by the bishop. And as he did not receive this, how could he receive the Holy Spirit?'" Pope Cornelius [regn. A.D. 251-253], To Fabius, fragment in Eusebius' Ecclesiastical History 6,43:14 (A.D. 251) .
"It is also necessary that he should be anointed who is baptized; so that, having received the chrism, that is, the anointing, he may be anointed of God, and have in him the grace of Christ. Further, it is the Eucharist whence the baptized are anointed with the oil sanctified on the altar. But he cannot sanctify the creature of oil, who has neither an altar nor a church; whence also there can be no spiritual anointing among heretics, since it is manifest that the oil cannot be sanctified nor the Eucharist celebrated at all among them. But we ought to know and remember that it is written, 'Let not the oil of a sinner anoint my head,' which the Holy Spirit before forewarned in the Psalms, lest any one going out of the way and wandering from the path of truth should be anointed by heretics and adversaries of Christ." Cyprian, To Januarius, Epistle 70/69:2 (A.D. 255) .
"They who are baptized must after Baptism be anointed with the heavenly chrism, and be partakers of the Kingdom of Christ." Council of Laodicea, Canon 48 (A.D. 343-381) .
"But a gate has been opened for seeking peace, whereby the mist has lifted from the reason of the multitude; and light has dawned in the mind; and from the glistening olive, fruits are put forth, in which there is a sign of the sacrament of life, by which Christians are perfected, as well as priests and kings and prophets. It illuminates the darkness, anoints the sick, and leads back penitents in its secret sacrament." Aphraates, Treatises, 23:3 (A.D. 345) .
"But beware of supposing this to be plait ointment. For as the Bread of the Eucharist. after the invocation of the Holy Ghost, is mere bread no longer, but the Body of Christ, so also this holy ointment is no more simple ointment, nor so to say common, after invocation, but it is Christ's gift of grace, and, by the advent of the Holy Ghost, is made fit to impart His Divine Nature. Which ointment is symbolically applied to thy forehead and thy other senses; and while thy body is anointed with the visible ointment, thy soul is sanctified by the Holy and life-giving Spirit." Cyril of Jerusalem, Catechetical Lectures (On Chrism), 21:3 (A.D. 350) .
"You may effect in this chrism a divine and heavenly operation, so that those baptized and anointed in tracing with it of the sign of the saving cross of the Only-begotten, through which cross Satan and every adverse power is turned aside and conquered, as if reborn and renewed through the bath of regeneration, may be made participants in the gift of the Holy Spirit, and confirmed by this seal, may remain firm and immovable, unharmed and inviolate." Serapion of Thmuis, Prayer over Chrism, 25:1 (A.D. 350) .
"'And your floors shall be filled with wheat, and the presses shall overflow equally with wine and oil.'... This has been fulfilled mystically by Christ, who gave to the people whom He had redeemed, that is, to His Church, wheat and wine and oil in a mystic manner...the oil is the sweet unguent with which those who are baptized are signed, being clothed in the armaments of the Holy Spirit." Ephraim, On Joel 2:24 (ante A.D. 373) .
Don't you know that the laying on of hands after baptism and then the invocation of the Holy Spirit is a custom of the Churches? Do you demand Scripture proof? You may find it in the Acts of the Apostles. And even if it did not rest on the authority of Scripture the consensus of the whole world in this respect would have the force of a command. For many other observances of the Churches, which are due to tradition, have acquired the authority of the written law, as for instance the practice of dipping the head three times in the layer, and then, after leaving the water, of tasting mingled milk and honey in representation of infancy; and, again, the practices of standing up in worship on the Lord's day, and ceasing from fasting every Pentecost; and there are many other unwritten practices which have won their place through reason and custom. So you see we follow the practice of the Church, although it may be clear that a person was baptized before the Spirit was invoked. Jerome, Against the Luciferians, 8 (A.D. 379) .
"And then remember that you received the seal of the Spirit; the spirit of wisdom and understanding, the spirit of counsel and strength, the spirit of knowledge and godliness, and the spirit of holy fear, and preserved what you received. God the Father sealed you, Christ the Lord strengthened you, and gave the earnest of the Spirit in your heart, as you have learned in the lesson from the Apostle." Ambrose, On the Mysteries, 7:42 (A.D. 391) .
"He would likewise be permitting this to the Apostles alone? Were that the case,He would likewise be permitting them alone to baptize,them alone to baptize, them alone to Confer the Holy Spirit...If, then, the power both of Baptism and Confirmation, greater by far the charisms, is passed on to the bishops..." Pacian, Epistle to Sympronian, 1:6 (A.D. 392) .
'For indeed the word of Ecclesiastes says true; your heretic is no living man, but 'bones,' he says, 'in the womb of her that is with child'; for how can one who does not think of the unction along with the Anointed be said to believe in the Anointed? 'Him,' says (Peter), 'did God anoint with the Holy Spirit.' These destroyers of the Spirit's glory, who relegate Him to a subject world, must tell us of what thing that unction is the symbol. Is it not a symbol of the Kingship? And what? Do they not believe in the Only-begotten as in His very nature a King? Men who have not once for all enveloped their hearts with the Jewish 'vail' will not gainsay that He is this. If, then, the Son is in His very nature a king, and the unction is the symbol of His kingship, what, in the way of a consequence, does your reason demonstrate? Why, that the Unction is not a thing alien to that Kingship, and so that the Spirit is not to be ranked in the Trinity as anything strange and foreign either. For the Son is King, and His living, realized, and personified Kingship is found in the Holy Spirit, Who anoints the Only-begotten, and so makes Him the Anointed, and the King of all things that exist. If, then, the Father is King, and the Only-begotten is King, and the Holy Ghost is the Kingship, one and the same definition of Kingship must prevail throughout this Trinity, and the thought of "unction" conveys the hidden meaning that there is no interval of separation between the Son and the Holy Spirit. For as between the body's surface and the liquid of the oil nothing intervening can be detected, either in reason or in perception, so inseparable is the union of the Spirit with the Son; and the result is that whosoever is to touch the Son by faith must needs first encounter the oil in the very act of touching; there is not a part of Him devoid of the Holy Spirit. Gregory of Nyssa, On the Holy Spirit, 16 (ante A.D. 394) .
"But thou shalt beforehand anoint the person with the holy oil, and afterward baptize him with the water, and in the conclusion shall seal him with the ointment; that the anointing with oil may be the participation of the Holy Spirit, and the water the symbol of the death of Christ, and the ointment the seal of the covenants. But if there be neither oil nor ointment, water is sufficient both for the anointing, and for the seal, and for the confession of Him that is dead, or indeed is dying together with Christ. Apostolic Constitutions, 7,2:22 (A.D. 400) .
"Why, therefore, is the Head itself, whence that ointment of unity descended, that is, the spiritual fragrance of brotherly love,--why, I say, is the Head itself exposed to your resistance, while it testifies and declares that "repentance and remission of sins should be preached in His name among all nations, beginning at Jerusalem"? And by this ointment you wish the sacrament of chrism to be understood, which is indeed holy as among the class of visible signs, like baptism itself..." Augustine, Letters of Petilian the Donatist, 2,104:239 (A.D. 403) .
"That this power of a bishop, however, is due to the bishops alone, so that they either sign or give the Paraclete the Spirit...For to presbyters it is permitted to anoint the baptized with chrism whenever they baptize...but with chrism that has been consecrated by a bishop; nevertheless it is not allowed to sign the forehead with the same oil; that is due to the bishops alone when they bestow the Spirit, the Paraclete." Pope Innocent [regn. A.D. 401-417], To Decentius, 3 (A.D. 416) .
"The living water of holy Baptism is given to us as if in rain, and the Bread of Life as if in wheat, and the Blood as if in wine. In Addition to this there is also the use of oil, reckoned as perfecting those who have been justified in Christ through holy baptism." Cyril of Alexandria, Commentary on the Minor Prophets, 32 (A.D. 429) .