On the covenants in the initiation of becoming a Christian

Circumcision is part of the covenant process in the old testament, and is replaced by Paul’s spiritual interpretation of sacrifice in the new testament. I would have thought this covenant is different from the new covenant. I take it that Saint Paul still regards this covenant as relevant to Christians, it’s just he wants reinterpretation. I am therefore confused, as this leads me to think there are two covenants (the new covenant, and the covenant being reinterpreted) that should be taken to be Christian. Would you be so kind as to resolve this issue? If either of the covenant’s do need to be taken, how should they be formally done? Thank you all so much. As "I am planning on becoming a member of the Church of Scotland" I seek answers from those in that Protestant and Presbyterian 'stable'.