make a believer of
make a believer (out) of (one)
To convince or persuade one to believe or have faith in someone or something. I was really skeptical of his supposed breakthrough, but the demonstration he gave us made a believer of me. Their absolute dominance in the first game of the tournament made believers out of the entire city—now everyone is throwing their support behind the home team!
make a believer (out) of someone
to convince someone decisively about something. The game they played made a believer out of me; from now on, I'm betting on them. It was an interesting discussion, but it did not make a believer out of me.