This letter feels like a letter from a father to a son. In the greeting Paul says he longs to see Timothy so his joy might be full. . . and at the same time remembers him with weeping. The juxtaposition of joy and sadness in the life of God's servant is stunning, yet standard. They always go together.
Timothy had the privilege of being a 3rd generation Christian. His mother and grandmother passed on to him an unhypocritical faith. Because of their influence he had to fire up the gift of God in him. Were they to soft with him? Clearly he struggled with timidity and deeded to be reminded God gave him a spirit of power and love and a sound mind. Three times it says, "Don't be ashamed."
The confidence Paul is encouraging is found in his salvation which is, in turn, rooted in the eternal purpose of God. The strong language about God's purpose and determination in both his salvation and selection for service takes away an option to cave in or give up. We need to not be so afraid of strong language about the initiative of God in the lives of his people.