Circumcision vs Baptism
Where in the Bible can you find support for the idea that baptism is the NT equivalent of circumcision in the OT? You can find a lot on support for infant baptism, but not much on that particular point, and if you can't prove that baptism follows on from circumcision, it makes it a lot harder to support the infant baptism, because the main arguments for it rest on the assumption that baptism follows on from circumcision.
Though the reasons to baptise the children of the covenant-church are various, one of them is indeed that in the NT we find no more the requirement to circumcise believers and their children. In the OT any male non-Jew who desired to join Israel, needed to be circumcised to receive the token of his surrender to the covenant revelation of Jehovah. Think of how God came against Moses and was ready to slay him because he had omitted to circumcise his own children for many years to avoid war with his wife.
In the NT we find the example of Cornelius in Acts 10. He was a "god-fearer" which meant that he worshipped the God of Israel but hadn't become a formal Jew through circumcision. He didn't follow the Jews in all their rituals. If he had lived earlier, maybe eventually he might have because not being circumcised stood as a barrier between him and the God of Israel. Only those circumcised were regarded as being full members of the covenant community. But in his case he didn't because Peter preached to Him the Gospel. After having been filled with the Holy Ghost, he is baptised but not circumcised.
In the NT meeting in Acts 15, we don't read that Gentiles who repented and believed the Gospel needed to be circumcised. From other church history sources we can observe that circumcision entirely disappeared from the early Christian church. In Galatians Paul opposed false (Jewish) teachers who were insisting that Gentile believers had to be circumcised. Especially in Gal. 5:6 and 6:15 the apostles finishes off with this Jewish demand. Also in the letter to Colossians, where the same pressure was placed on believers by Jewish teachers, Paul wrote Col. 2:11-12. (I trust you have looked up the verse...) Paul actually tells them, "You are already circumcised in your heart (the dying of the old man and the rising of the new man) which is pictured beautifully in your baptism."
Another line of reasoning is that the "blood sacraments" of the OT are entirely fulfilled in the blood of the Lord Jesus. Both the circumcision and the pass-over looked ahead to Christ's shedding of His own blood to atone for sins. As this has taken place, we now commemorate His sacrifice sacramentally without the actual shedding of blood.
The fact that in the NT both male and female are equal in the participation of NT blessings is another argument for the abolishing of circumcision. Women were not circumcised in the OT but are baptised in the NT. I don't know exactly the reasons for that but have a few interesting thoughts. But that's not part of your question.
Lastly, there are many ways in which circumcision and baptism are alike. I found this helpful comparison in the book by Robert Booth showing that although there was a change in outward form, there was no change in meaning when baptism replaced circumcision as the sign and seal of the covenant of grace.
|Circumcision and Baptism||References|
|♦ both are initiatory rites||Gen. 17:10-11; Matt. 28:19; Acts 2:38-39; 8:12-13|
|♦ both signify an inward reality||2:28-29; Col. 2:11-12; Phil. 3:3|
|♦ both picture the death of old man of sin||Rom. 6:3-7; Col. 2:11-12|
|♦ both represent repentance||Jer. 4:4; 9:25; Lev. 26:40-41; Acts 2:38|
|♦ both represent regeneration||Rom. 2:28-29; Titus 3:5|
|♦ both represent justification by faith||Rom. 4:11-12; Col. 2:11-14|
|♦ both represent a cleansed heart||Deut. 10:16; 30:6; Is. 52:1; Acts 22:16; Titus 3:5-7|
|♦ both represent union and communion with God||Gen. 17:7; Ex. 19:5-6; Deut 7:6; Heb. 8:10|
|♦ both indicate citizenship in Israel||Gen. 17:4; Gal. 3:26-29; Eph. 2:12-13; 4:5|
|♦ both indicate separation from world||Ex. 12:48; 2 Cor. 6:14-18; Eph. 2:12|
|♦ both can lead to either blessings or curses||Rom. 2:25; 1 Cor. 10:1-12; 11:28-30|