In Deuteronomy, Moses recounts God’s establishment of His covenant with His people and the giving of the Ten Words. He walks them through a covenant renewal ceremony, rehearsing Israel’s disobedience and urging them to trust God, to obey, and to remember what God has done for them in their deliverance from bondage in Egypt. Moses had been part of that deliverance; he had led the people for forty years, shepherding them, correcting them, pleading for them, even offering his own life for theirs. As one of his final acts, and as part of this covenant renewal, Moses taught the people a song as a witness to the covenant and a reminder to this and future generations to keep the words of Yahweh, warning them of the consequences of disobedience and assuring them of the faithfulness and vindication of God, their Rock.
Likewise, John, in his first epistle, writes to remind us to keep the words of God. He says that keeping the commandments of God is walking in the light, because God is light. But this is not just an individual piety; our walking in the light necessarily includes our relationship with our brother. We can say we walk in the light, giving the appearance of meticulously keeping the commandments, but if we do not love each other, John says, we have missed the whole thing. The love of God is not in us and we walk in darkness. But if we do love each other, it is evident to us and to the world that we are in Christ and He is in us. We walk in the light, our sins are forgiven, we know the Father, and we have overcome the evil one.