"But we have this treasure in earthen vessels, that the exceeding greatness of the power may be of God, and not from ourselves;" (ASV)
Have you ever felt that you're not good enough to accomplish great things for God? Have you looked at other Christians, and felt inferior? Have you not done something you felt God was calling you to do because you were afraid you might fail? This feeling is more common than most people realize. Even Moses, when confronted with the burning bush, worried he could not do what God was telling him to do. "Moses said to the LORD, 'O Lord, I have never been eloquent, neither in the past nor since you have spoken to your servant. I am slow of speech and tongue.'" (Exodus 4:10 ASV) "But Moses said, 'O Lord, please send someone else to do it.'" (Exodus 4:13 ASV) Of course, God knew who He was choosing then, and He knows our strengths and weaknesses now.
The problem in thinking "I'm not good/smart/strong/well-spoken enough to do God's work" is that we assume that we will be working under our own power. God does not call us to do anything that He will not help us in doing. It's not our own power, but His that we must rely on. No human is good enough to do the things God would have done on this earth. But God working through us is powerful enough to do anything He wants. Just as God told Moses, there on the mountain, "Now go; I will help you speak and will teach you what to say," (Exodus 4:12 ASV) He offers the same aid to anyone who is doing His work.
If the miracles of God could be performed by men, they would cease to be miracles of God. What surprise would it be if a star athlete competed in a triathalon? But if a crippled man were to finish the race, people would know that it had to be by a power that is not his own. In the same way, God uses us in all of our weakness and frailty, to brings glory to His name. He does not call great men to greatness. He calls normal men and makes them great.
So the next time you feel God's calling in your heart and you think "but I can't do that," stop for a moment and remember Moses. Remember that God does not want you to do great works, but rather He wants to do great works through you.