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I don't have great faith. I have a great God!

When you have a great God, you don't need a lot of faith - just enough to take Him at His word. … The size of someone's faith is not important. What is important is the will of God.

What the Lord is saying is that, if it's God's will (as the initiator) that you move the tree (Luke 17:6), all it takes for you to move it (as the responder) is the faith to speak the command.

The real question is always, 'What has God said?'
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Bob George, "Classic Christianity"