Oswald Chambers writes in So Send I You the following excerpt. God called, but He had to prepare us for the ministry He has for us.

I know God called me to be a missionary—but it was a long time ago, and I suppose I was mistaken, for look at me now, something in an office.” You know you were not mistaken. The call was right, but you were not ready for it. God has to season us, there has to be a time of humiliation before the vision is turned into verity. We have to learn not only how useless we are, but how marvellously mighty God is. “Many are called, but few prove the choice ones.”

Think of the enormous leisure of God! He never is in a hurry. We are in such a frantic hurry. We get down before God and pray, then we get up and say, “It is all done now,” and in the light of the glory of the vision we go forth to do the thing. But it is not real, and God has to take us into the valley and put us through fires and floods to batter us into shape, until we get into the condition in which He can trust us with the reality of His recognition of us.

 Oswald Chambers, So Send I You: The Secret of the Burning Heart (Hants UK: Marshall, Morgan & Scott, 1930).