Christ told John not to stop a man who was doing miracles in His name who was not a follower.
Mark 9:38
John said to Him, "Teacher, we saw someone driving out demons in
Your name, and we tried to stop him because he wasn't following us." 39
"Don't stop him," said Jesus, "because there is no one who will perform a
miracle in My name who can soon afterwards speak evil of Me. 40 "For he
who is not against us is for us. 41 "For whoever gives you a cup of water
to drink because of your name as followers of Christ, truly I say to you,
he will not lose his reward. 42 "Whoever causes one of these little ones
who believe to stumble, it would be better for him if, with a heavy
millstone hung around his neck, he had been cast into the sea
Paul faced the fact that there were preachers that were preaching the Gospel for their own personal motives.
Philippians 1:12
Now I want you to know, brethren, that my circumstances
have turned out for the greater progress of the gospel, 13 so that my
imprisonment in the cause of Christ has become well known throughout the
whole praetorian guard and to everyone else, 14 and that most of the
brethren, trusting in the Lord because of my imprisonment, have far more
courage to speak the word of God without fear. 15 Some, to be sure, are
preaching Christ even from envy and strife, but some also from good will ;
16 the latter do it out of love, knowing that I am appointed for the
defense of the gospel ; 17 the former proclaim Christ out of selfish
ambition rather than from pure motives, thinking to cause me distress in
my imprisonment. 18 What then ? Only that in every way, whether in
pretense or in truth, Christ is proclaimed ; and in this I rejoice. Yes,
and I will rejoice,
Reviewing both of the above passages it appears that both Paul and Christ agree that preachers who do not believe the gospel themselves, will, as long as they are preaching the correct gospel are in fact doing a service to Christ. It appears that Christ later on in Mark states that if they preach the wrong gospel or lead their sheep astray, they will be worse off than they were to start off with using the analogy of of the millstone around their neck and thrown into the sea.
Of course the trouble with this is that if you do not believe the gospel yourself than you are a tool of Satan. If you are tool of Satan you will stray from the Gospel and lead the others with you.
I think we can see other places in the Bible where God uses non-believers to further his desires. After all doesn't he tell us that all things work for good according to His purpose? Romans 8:28
IHS
JB