It begins as somewhat of a rhetorical question. Then from there on out, the person is stating his faith that God is his protector or guardian. This sentiment can be summed up by six words of the passage- m y help cometh from the Lord . Another idea expressed in the passage, is that of trust. Faith and trust might seem to be similar in a few ways, yet they really represent two distinct opinions.
Faith is a precursor to trust.
We must have faith in God and believe in him, and then we will trust him with our lives because he is the protector and guardian. That leader is God. Although not explicitly stated in the passage, sin is basically what God is protecting and guarding us from. Things such as the sun shall not smite thee by day, nor the moon by night are statements that refer to protection from something. Obviously, the only something that we would need that multitude of protection from is sin.