“Hear, O Israel: The Lord our God, the Lord is one. You shall love the Lord your God with all your heart and with all your soul and with all your might."The first part - verse 4 - is the Shema, the central confession of the faithful Jew. God is One. I think that means two things.
Firstly, God is unique. This does not necessarily mean that Deuteronomy is teaching a rigorous monotheism here; in fact, the book seems to maintain the reality in some sense of other gods and spiritual powers. Even when Moses affirms that "the Lord is God in heaven above and on the earth beneath; there is no other" the context implies a comparison with other 'gods'. The uniqueness of Yahweh, the God of Israel, is not simply a matter of alone-ness. Rather it is that none of the other 'gods' or powers or whatever you want to call them are this God, the Creator of all and the Redeemer of his people. He is unique. What other 'god' has ever tried to save a people out from the midst of another nation? What other 'god' has ever spoken to his people and entered into saving relationship with them? As we move towards the New Testament, we have to add: what other 'god' has humbled himself to human flesh and Calvary's cross to redeem a people for himself?
Martin Luther in his Large Catechism asks: "what does it mean to have god? Or what is God?" His answer is: "a god means that from which we are to expect all good and to which we are to take refuge in all distress". In other words, what you trust is your god. I think this helpfully illuminates the meaning of Deuteronomy. The Lord is God, the one and only; he is the one from whom we are to expect all to good, and in whom we are to take refuge in all distress. He and he only, because he is the only real Saviour.
But second, God is united. God is always himself. He is not in any sense divided. In this he stands in contrast with the ancient deities, who might appear differently in different sacred sites. He also stands in sharp contrast with us. We often find that we are divided against ourselves, hardly knowing what it is that we want or who it is that we really are. Not so God. He is always God. That means that he is always dependable, always the same. The Lord is One, and therefore he can be our God.
It's worth noting in passing that, theologically speaking, the fact that God is One is also the foundation of the church's doctrine of the Trinity. Because God is One, we can take Jesus absolutely seriously when he says that to see him is to see the Father. Wherever the Son is, there is the Father and the Spirit. Therefore, in Jesus, we have a true revelation of God, God without remainder.
Between verse 4 - the theological affirmation - and verse 5 - the instruction to Israel - there is an implied 'therefore'. Because God is One, you shall love him with all your heart, soul, and strength. The logic is simple: because he is the only god, in the sense discussed above - the only source of good and only refuge of our souls - he is to receive absolute loyalty, love, devotion. That could be terrifyingly totalitarian, and indeed it would be if any human being were to make such a claim on our loyalty. But to love God wholeheartedly does not eclipse the love of other things. Rather, it orders the love of created things, such that in loving God wholeheartedly we find ourselves loving other people and indeed all God's creation appropriately - and we find that our love for those created things flow back into love of the Creator. Because God is really God, the source and fountain of all good, he is not a black hole sucking in all our devotion and love, but the one in whom we really learn what it is to love in the first place.
And then again, because God is united, wholehearted love of him is the only way to bring our fragmented and sin-shattered lives together. "Unite my heart to fear your name", prays the Psalmist. Take, O God, this bundle of contradictions that I call myself, and, by orienting it around your great self, bring it to order and sense. God alone is great enough to be the sun at the centre of the solar system of your life. This is why in Jesus we see the only real example this fallen world has ever known of true humanity - life properly oriented, lived out of a centre in God which makes the disparate whole and the complex simple.