Here is a quote from his book One Thing You Can't Do in Heaven which hits the nail on the head:
A youth pastor in Atlanta once asked me, "Mark, why is it that when we are in church or in a good worship time, God is so big and Satan is so small; but when we go out witnessing, Satan is so big and God seems so small?"
I didn't have an answer.
He asked, "Did God change?"
"Of course not."
"That's right," he said. "God hasn't changed, but our perception of God has."That is an incredible truth. God is big wherever you go. Whether you witness in the bar section of town, at a music festival, or during spring break, don't ever forget how BIG your God is!"
You can learn more about Mr. Cahill and his Ministry here. And if you would like to order his books, you can do that here.