How was it decided which of the OT laws applied to Christians and which were no longer of concern?
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)