a. How does Saint Paul describe the unified body of Christians?

"For just as the body is one and has many members, and all the members of the body, though many, are one body, so it is with Christ. For in the one Spirit we were all baptized into one body. (12:12-13)

b. When and what was decided at the Council of Jerusalem?

It was decided at the Council of Jerusalem, which occurred five to eight years after Jesus' crucifixion, that Gentile Christians were free from the requirements of the Jewish Law.

A+B