The Lord is my strength and song - He is the source of strength to me; and he is the subject of my praise. There is no ground of praise in myself for anything that I have done, but all is due to him.
And is become my salvation - He has saved me. I live because he preserved me. So we shall be saved in heaven solely because he saves us, and there, more than can be possible here, we shall say, “God is our strength and our song, and is become our salvation.”