For God is my King of old - That is, the king, or ruler of his people. The people had acknowledged him as their king and ruler, and he had showed himself to be such. This is given as a reason why he should now interpose in their behalf. It is an argument, proper always to be urged, drawn from the faithfulness and unchangeableness of God.
Working salvation in the midst of the earth - Salvation for his people. The reference here particularly is to what he had done for his people in delivering them from bondage in Egypt, and conducting them to the promised land, as is stated in the following verses.