Regardless of the blessings and fruitfulness God grants to your service, there will be dark, heavy days when you would walk away from the Gospel ministry if it weren't for the bedrock of assurance that you are doing what God Himself has called you to do. Not having an unshakable sense of divine call is one of the main reasons why so few who begin to preach stay devoted to it for a lifetime. God's call to preach His Word is much more than a personal spiritual experience. When God calls a man to the Gospel ministry He calls him both internally and externally. Each of these twin features of God's call also has two aspects of its own.
Some people first become aware of the call originating through their reading of the Bible.
Many recall the powerful effect of a sermon or a book that initiated their sense of summons. Others will refer to the influence or example of a minister they admired. Regardless of the means through which God began to awaken you to His purpose, your encounters with the Scriptures should continually nourish and affirm the secret call. So the internal call should be a continuing aspiration. Without choosing to think about it you find it keeps surfacing in your thoughts.