If we preach the gospel solely out of mechanical obedience, without praying for and expecting God to convert those who hear we sin the sin of not preaching in faith.
I once had a graduate student who pastored a church in the area. We were talking about the different emphasis on our Sunday morning and Sunday evening services. He remarked that he often had people saved in the morning service but had never had anyone saved in the evening service.
I said, "You don't really expect people to get saved in the evening service, do you?
He said, "No, I guess not."
I responded with, "Well, there's your problem."
Matthew 21:22 "And all things, whatsoever ye shall ask in prayer, believing, ye shall receive."