That's because we don't have circumcision, so the concept is unfamiliar to us.
All we have are the promises that if anyone is in Christ they are a new creation; the old has gone, the new has come, 2 Corinthians 5:17.
That if we confess our sins God is faithful, will forgive us and make us clean, 1 John 1:9; and that we are reconciled to God in Christ, Romans 5:11, 2 Corinthians 5:18.
The Gospel - Good News - is that mankind sinned against God, deserved punishment and eternal separation from God, and could no nothing to reconcile ourselves to him - so God came to earth, in Jesus, and took our punishment for us. He lived, taught, died, was raised again, ascended back to his Father and sent his Spirit to live in us. God no longer lives in a building or a tabernacle but in the hearts of his children.
For the purposes of these forums, a person may only call themselves a Christian if they agree with, and believe, the Nicene Creed.