Here are some Bible references
The gift of the Holy Spirit from the beginning has been associated with baptism and has been given to Christians Acts 2:38 They asked Peter and the other apostles, “What are we to do, my brothers?” Peter [said] to them, “Repent and be baptized, every one of you, in the name of Jesus Christ for the forgiveness of your sins; and you will receive the gift of the holy Spirit.
Confirmation is part of our initiation as Christians Hebrews 6:1-2 Therefore, let us leave behind the basic teaching about Christ and advance to maturity, without laying the foundation all over again: repentance from dead works and faith in God, instruction about baptisms and laying on of hands, resurrection of the dead and eternal judgment.
In his sermon in Acts 2, Peter says this prophecy of Joel is fulfilled in the gift of the Holy Spirit Joel 3:1-2 It shall come to pass I will pour out my spirit upon all flesh. Your sons and daughters will prophesy, your old men will dream dreams, your young men will see visions. Even upon your male and female servants, in those days, I will pour out my spirit.
A startling communion is given by God's Spirit Romans 5:5 Hope does not disappoint, because the love of God has been poured out into our hearts through the holy Spirit that has been given to us.
Here are some references from the Catechism
Paragraphs 1294 and 1295 By Confirmation Christians, that is, those who are anointed, share more completely in the mission of Jesus Christ and the fullness of the Holy Spirit with which he is filled, so that their lives may give off "the aroma of Christ." By this anointing the confirmand receives the "mark," the seal of the Holy Spirit. A seal is a symbol of a person, a sign of personal authority, or ownership of an object. Hence soldiers were marked with their leader's seal and slaves with their master's. A seal authenticates a juridical act or document and occasionally makes it secret.
Paragraph 1299 In the Roman Rite the bishop extends his hands over the whole group of the confirmands. Since the time of the apostles this gesture has signified the gift of the Spirit. The bishop invokes the outpouring of the Spirit in these words:
- All-powerful God, Father of our Lord Jesus Christ,
by water and the Holy Spirit
you freed your sons and daughters from sin
and gave them new life.
Send your Holy Spirit upon them
to be their helper and guide.
Give them the spirit of wisdom and understanding,
the spirit of right judgment and courage,
the spirit of knowledge and reverence.
Fill them with the spirit of wonder and awe in your presence.
We ask this through Christ our Lord.
Paragraph 1303b (quoting St. Ambrose) Guard what you have received. God the Father has marked you with his sign; Christ the Lord has confirmed you and has placed his pledge, the Spirit, in your hearts.