18 Then after three years I went up to Jerusalem to see Peter, and remained with him fifteen days.
19 But I saw none of the other apostles except James, the Lord’s brother.
20 (Now concerning the things which I write to you, indeed, before God, I do not lie.)
It was three years after his salvation before Paul saw any of the apostles.
see (v.18) = visit for the purpose of getting acquainted.
Peter was the leader of the 12 apostles.
fifteen days (v.18) — Recorded in Acts 9:26-30 and Acts 22:17-21. Much occurred on this visit that Paul doesn’t mention in Galatians, but it wasn’t germane to his point — that he didn’t receive his message from the 12 apostles.
none of the other apostles (v.19) = other of the same type
Barnabas brought Paul to see the apostles, but this only consisted of Peter and James (who is referred to as an apostle in a greater sense but who wasn’t one of the 12). The James here was the writer of the book of James and a prominent leader in the Jerusalem church.
Paul only stayed 15 days because the Jews were seeking to kill him and the Lord ordered him to leave Jerusalem, where his message wouldn’t be accepted, and go to the Gentiles.
The point that Paul was making about the short duration and limits of his visit to Jerusalem were so important that he followed it up with a strong statement, before God, that he wasn’t lying. This was to counter the claims the Judaizers were making to the Galatians about Paul’s ministry, but also apply to us as evidence of the uniqueness of his gospel.