Hello Jim,
@Jim6
Welcome to Puritan Board!
The context of what Peter said is this, after the crowd in Jerusalem were convicted of being a party to the crucifixion of their Lord and Christ,
Now when they heard this, they were pricked in their heart, and said unto Peter and to the rest of the apostles, Men and brethren, what shall we do? Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost. For the promise is unto you, and to your children, and to all that are afar off, even as many as the Lord our God shall call (Acts 2:37, 38, 39) [emphasis added]
This was the beginning of the change of the covenant sign and seal from circumcision to baptism, so that the women and even infant girls could receive the covenant seal. Infant boys always could, it being mandated for them on the eighth day. Not all who receive the covenant sign are the elect (as Scripture abundantly shows), but for the sake of the elect all are baptized.
As we are the seed of Abraham – "And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise" (Gal 3:29KJV) – the requirement to present our children to the LORD to circumcise their hearts remains the same (cf Deut 30:6KJV: "And the LORD thy God will circumcise thine heart, and the heart of thy seed, to love the LORD thy God with all thine heart, and with all thy soul, that thou mayest live.")
The initial command to Abraham's seed was given in Gen 17:10KJV and Gen 17:14KJV.
That baptism is the New Covenant sign what Old Covenant circumcision signified is seen here:
In whom [Christ] also ye are circumcised with the circumcision made without hands, in putting off the body of the sins of the flesh by the circumcision of Christ: Buried with him in baptism, wherein also ye are risen with him through faith in the working of God, who hath raised him from the dead. And you, being dead in your sins and the uncircumcision of your flesh, hath he quickened [made alive] together with him, having forgiven you all trespasses (Col 2:11-13)
We adults are subject to the same command to present our children to receive the sign and seal of the covenant as were the saints of old. When the elect among them are actually regenerated is in the hands of God.
I hope this clarifies somewhat.