The call is not to be taken lightly. For a person to possess knowledge is not enough. He must be sure that he is properly called. Those who operate without a proper call seek no good purpose. God does not bless their labors. They may be good preachers, but they do [not] edify. Many of the fanatics of our day pronounce words of faith, but they bear no good fruit, because their purpose is to turn men to their perverse opinions. On the other hand, those who have a divine call must suffer a good deal of opposition in order that they may become fortified against the running attacks of the devil and the world.
This is our comfort in the ministry, that ours is a divine office to which we have been divinely called. Reversely, what an awful thing it must be for the conscience if one is not properly called. It spoils one’s best work. When I was a young man I thought Paul was making too much of his call. I did not understand his purpose. I did not then realize the importance of the ministry. I knew nothing of the doctrine of faith because we were taught sophistry instead of certainty, and nobody understood spiritual boasting. We exalt our calling, not to gain glory among men, or money, or satisfaction, or favor, but because people need to be assured that the words we speak are the words of God. This is no sinful pride. It is holy pride.
Martin Luther, Commentary on Galatians (Kindle Edition, location 87)