Albert Barnes Commentary Acts 18:21

"but taking his leave of them, and saying, I will return again unto you if God will, he set sail from Ephesus." — Acts 18:21 (ASV)

Keep this feast. The Passover is probably referred to here. Why he was so anxious to celebrate that feast at Jerusalem, the historian has not informed us. It is probable, however, that he wished to meet as many of his countrymen as possible, and, if practicable, to remove the prejudices that had everywhere been raised against him (Acts 21:20–21). Perhaps, also, he supposed that there would be many Christian converts present, whom he might also meet.

But I will return, etc. This he did (Acts 19:1), and remained there three years (Acts 20:31).