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 confirm and 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.
From: Tradition/ Words of the Church Fathers
"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) .
“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) .