Sounds like a contradiction, does it not? But I only meant unpaid in this world. So what enterprise are we thinking about? Most professions are sealed up and not available to "amateurs". But there is one open to Christians. Imagine yourself becoming a professional grade Bible teacher without spending time and money to attend a theological "cemetery".
It's been done. Blackstone, known as W.E.B., did it. A man without credentials wrote a book on a Bible theme that no one can contradict, though some would like to. He wrote Jesus is Coming Again, a jewel of a book.
He has been called, along with Scofield, a Zionist. Kind of like saying the polar bear is a tropical animal. The opposite is the case. These two men did teach that Israel will be restored, but after the church is complete and raptured.
Point being, that God can use uncredentialed people. There has never been such a time as now, when you can become a pro in Bible teaching. God calls and equips teachers. The resources are there to enable you to do a good job. Your pay will be waiting for you in heaven.