I should have asked what Orthodox believe about the old and new covenants and what makes the new one, new but i thought that might be too broad and off topic even if relevant to Peter's link of circumcision and baptism.
I do think it's relevant.
4 Now this I say, that no man may deceive you by loftiness of words. 5 For though I be absent in body, yet in spirit I am with you, rejoicing, and beholding your order and the steadfastness of your faith which is in Christ. 6 As therefore you have received Jesus Christ the Lord, walk in him: 7 Rooted and built up in him and confirmed in the faith, as also you have learned: abounding in him in thanksgiving. 8 Beware lest any man cheat you by philosophy and vain deceit: according to the tradition of men according to the elements of the world and not according to Christ. 9 For in him dwells all the fullness of the Godhead corporeally. 10 And you are filled in him, who is the head of all principality and power. 11 In whom also you are circumcised with circumcision not made by hand in despoiling of the body of the flesh: but in the circumcision of Christ. 12 Buried with him in baptism: in whom also you are risen again by the faith of the operation of God who has raised him up from the dead. 13 And you, when you were dead in your sins and the uncircumcision of your flesh, he has quickened together with him, forgiving you all offences: 14 Blotting out the handwriting of the decree that was against us, which was contrary to us. And he has taken the same out of the way, fastening it to the cross. 15 And despoiling the principalities and powers, he has exposed them confidently in open show, triumphing over them in himself.http://newadvent.org/bible/col002.htm
Yes, circumcision of the heart is now today.
Baptism has always been practiced, it's not a replacement.
If Baptism is the same and just about repentance both OT and NT, then why did Paul bother to rebaptized those who only had the Baptism of John?
Acts 19:19 And it happened, while Apollos was at Corinth, that Paul, having passed through the upper regions, came to Ephesus. And finding some disciples 2 he said to them, “Did you receive the Holy Spirit when you believed?”
So they said to him, “We have not so much as heard whether there is a Holy Spirit.”
3 And he said to them, “Into what then were you baptized?”
So they said, “Into John’s baptism.”
4 Then Paul said, “John indeed baptized with a baptism of repentance, saying to the people that they should believe on Him who would come after him, that is, on Christ Jesus.”
5 When they heard this, they were baptized in the name of the Lord Jesus.