" . . . Let not your heart be troubled, neither let it be afraid."
The most fearless people are those who are the most naturally timid and afraid. This seems odd, but it is true. The reason for this is because Christ instills confidence in those who--in themselves--would lack it the most. This boldness is a result of faith. The believer says, "OK, God commands me not to fear--therefore, I will determine, in Christ, to be fearless."
The apostles themselves were good examples of this. Some of them were initially falsely self-confident (like Peter); but all of them were really fearful by nature. Only after the power of Pentecost were they filled with so much divinely-inspired assurance, that nothing could stop them.
[Puritan quote of the day: "The Holy Ghost is content to dwell in smoky, offensive souls." --Richard Sibbes, in "The Bruised Reed"]