Then after three years I went up to Jerusalem to see Peter, and abode with him fifteen days.
All Commentaries on Galatians 1:18 Go To Galatians 1
Jerome
AD 420
He who had prepared himself for so long a time did not need any long instruction. And, though it seems excessive to some to investigate numbers in Scripture, yet I think it not beside the point to say that the fifteen days that Paul spent with Peter signifies [in late Judaic piety] the fullness of wisdom and the perfection of doctrine, seeing that there are fifteen psalms in a psalter and fifteen steps by which people go up to sing to God. .