Charles Ellicott Commentary Acts 19:18

"Many also of them that had believed came, confessing, and declaring their deeds." — Acts 19:18 (ASV)

And many that believed.—More accurately, many of those that had believed. The word is probably used, as in Acts 19:2, for the whole process of conversion, including baptism, confession in this instance following that rite, instead of preceding it. The words do not definitely state whether the confession was made privately to St. Paul and the other teachers, or publicly in the presence of the congregation; but the latter is, as in the confession made to the Baptist, much more probable. (See Note on Matthew 3:6.)

The feeling of a vague awe at this contact with the Unseen in some, the special belief in Christ as the Judge of all men in others, roused conscience into intense activity. The sins of their past lives returned to their memories, and it was a relief to throw off the burden by confessing them.