John Gill Commentary Judges 17:12

"And Micah consecrated the Levite, and the young man became his priest, and was in the house of Micah." — Judges 17:12 (ASV)

And Micah consecrated the Levite
Installed him into, and invested him with the priestly office; in like manner he had consecrated his son before, by filling his hand with sacrifices; see (Judges 17:5)

and the young man became his priest ;
and did the work and office of one; this was a very daring piece of presumption in them both; in Micah, to take upon him to consecrate a priest, who was himself of the tribe of Ephraim; and in the young man, to suffer himself to be put into such an office, which did not belong to him, for though every priest was a Levite, or of the tribe of Levi, yet every Levite had not a right to be a priest, only those who were of the family of Aaron:

and was in the house of Micah ;
and continued there.