As an appendix to the last Q&A, which addressed the meaning of 1 John 4:18, let’s consider another application of the same principle (mature love casts out fear), which I believe is legitimate given the larger context of John’s epistle concerning brotherly love. That is, that growth in love for others leads to the lessening of fear in relationships.
For example, the “introvert” (for want of a better word) overcomes his fear of people by choosing, in obedience to God, to exercise his will in spite of his strong, fearful emotions. He does this by considering another person as more important than himself (Phil. 2:3) and, consequently, walks across the church foyer to speak to a total stranger. His choice to love that other person has overcome his fear of them or his fear of rejection. The more he does this (practices others-focused love) the more he is being perfected, made complete in Christlikeness.
As a pastor and preacher, I must overcome fear every time I step into the pulpit. Those who know me well know that if I had planned my life on my own then I would have remained in a quiet corner somewhere, content. I would never have chosen the pulpit! But my love for God translates into obedience to His call to preach and my God-given love for my congregation overcomes my fear of being in front of people. Perfect love (or at least God’s love being perfected in me) casts out (or at least diminishes) my fear.
[P.S. This post is limited to an application of 1 John 4:18 to the fear struggle of the "introvert." I am not saying that every trait of an "introvert" is sinful. God has wired each of us differently for His unique purposes in the body of Christ. I will try to deal with that angle in a future post.]