In God, whose word I praise, in God I trust; I will not be afraid. What can mortal man do to me?
This phrase is used twice in this short passage. Once in verse 4, and once in verses 10-11. David is being pursued and attacked, and the study help in my Bible says that he had been seized by the Philistines when he wrote this.
The way I see it, there are 3 main things to note about this little phrase. First, David begins with remembering who God is. "In God, whose word I praise," This is the God whose very words, thoughts, and plans are worthy of praise. By His word the world came into being. The Lord, whose word is so powerful, deserves praise.
Then David moves from praising God's word to trusting in Him. If God is so mighty, so powerful, that He needs but speak and the world comes into existence, then He is surely powerful enough to handle the situation that we find ourselves in. If we start with praise for who God is, remembering what He has done, that perspective will lead us to trust in Him. And if we trust in Him, we need not be afraid, for our trust is no longer in ourselves, but in Him.
And once you trust in the Lord, as David has done in this passage, why should you fear? "What can mortal man do to me?" Our hope in the Lord is eternal, and we know that His plan is eternal. Men can only take from us what is here on this earth, but that is not where our treasure is stored. They cannot take the Lord from us, nor our eternal security. If we die here on earth we go to be with the Lord, so really, what can man do to us? Nothing. Not if our hope is in the Lord.