How was it decided which of the OT laws applied to Christians and which were no longer of concern?
It seems that if they were NOT related to Faith and Morals that was the guiding principle.
Only those laws that were related to customs/dietiry etc where abolished.
The circumcision is a prime example.
The jews were welcomed into their faith with that act.
Christians are welcomed with Baptism, circumcision was NOT necessary.
It was revealed to Peter that certain Jewish customs were no longer needed.
9 About noon the following day as they were on their journey and approaching the city, Peter went up on the roof to pray. 10 He became hungry and wanted something to eat, and while the meal was being prepared, he fell into a trance. 11 He saw heaven opened and something like a large sheet being let down to earth by its four corners. 12 It contained all kinds of four-footed animals, as well as reptiles and birds. 13 Then a voice told him, “Get up, Peter. Kill and eat.”
14 “Surely not, Lord!” Peter replied. “I have never eaten anything impure or unclean.”
15 The voice spoke to him a second time, “Do not call anything impure that God has made clean.”
16 This happened three times, and immediately the sheet was taken back to heaven.
And Paul says:
[L]et no one pass judgment on you in questions of food and drink or with regard to a festival or a new moon . . . These are only a shadow of what is to come; but the substance belongs to Christ . . . Why do you submit to regulations, “Do not handle, Do not taste, Do not touch” (referring to things which all perish as they are used), according to human precepts and doctrines? These have indeed an appearance of wisdom in promoting rigor of devotion and self-abasement and severity to the body, but they are of no value in checking the indulgence of the flesh. (Col. 2:16-17; 20-23)
The Lord Jesus delegated His authority (given to Him by the Father) to the Apostles (Matt. 28:18). They and their successors exercise this authority in Council, or in the person of their head, the Pope.
The first exercise of this authority is, of course, Peter’s decision to replace Judas (Acts 2). The first council is described in Acts 15, when the Council expressly determines which parts of the Jewish Law to require of the Gentile Christians. Later (post-Biblical era) Councils continued this process, relying on the guidance of the Holy Spirit that Jesus had promised (John 14:26).