"Before sailing from Cenchreae [Paul] had his head shaved because of a vow he had taken." (Acts 18.18 GNB)

Paul's vow, according to some commentaries, might be a Jewish Nazarite vow. Luke, the author of Acts, did not mention what Paul's vow was about nor why. Only that he had taken a vow. And we do not know the rest of the story of this particular vow.

Why did Luke write about this bit of the story without the rest? Did Paul keep it a secret even to Luke? Or did Luke know and somehow decided not to explain it? Or maybe it was so obvious to Luke's intended readers in those days that he did not need to explain it.

Either way, some things between me and God can remain just between me and God, I suppose. What is important is the commitment to carry it through.