Paul is teaching that our allegiance to God is not merely like that of a slave to his master. It’s a relationship in which we draw strength from God, and are led by Him, like a child draws strength from and is led by His parents — and this is what makes us children of God. As Latter-day Saints, we can think of it this way: by making and keeping covenants with God, we can take up our role as the children of God (as something we do, rather than something we merely are).