Yes an atheist priest can perfectly minister to a believing congregation and miracles can happen in that congregation. Miracles depend on the faith of the believer, not that of the officiant. A bartender who never takes alcohol can serve alcohol to his clients. What is necessary is that the priest believes he is doing the good work. The congregation needs faith and it helps them. It would be evil to deny them such a service in the name of his lack of faith. - Bangambiki Habyarimana.

If you are a pastor there is no way you can lose faith without losing face, you are considered the devil incarnate by your former congregation.