I was once an atheist, and no amount of preaching without proof could have brought me to my faith. It has nothing to do with an agnostic or even an atheist having a closed mind. It has everything to do with the "preacher" not having a valid argument.
If he doesn't have proof, he should just drop the preaching and maybe talk about the wonderful ways his life is fulfilled in his faith. After that, there's plenty of time to work with a nice organization like Orthodox Christian Charities :)
His justification for closing his mind is irrelevant - this is a Religion Forum. Preaching happens.
When he is unconvinced or doubtful, he would be treated like an agnostic.