"I will not be afraid of ten thousands of people who have set themselves against me all around."
What gives a believer this kind of confidence? Assurance. David, though a sinner, knew he was loved, blessed, and cherished by God. Therefore, it did not really matter that his son Absalom was chasing him all around the countryside. The result was a foregone conclusion: God would be with David, and he (David) knew it.
Unless we come to realize the blessed state of assurance of God's favor, we will always fear man (and everything else). And why not? We have no solid belief that we are in the Lord's grace. Are you a child of the King, as a member of Christ's church? If so, then seek to make your calling and election sure (2 Pet. 1:10).
[Puritan quote of the day: "[God] knows that divine manifestations of love will most awaken, quicken, and engage the soul to ways of piety and sanctity." --Thomas Brooks, in "Heaven on Earth"]