In Institutes 2.8.11, John Calvin gives an account of some of the reasons for which the moral law as summarized in the Decalogue is divided into two tables.
In brief, the Law is a seamless whole, and its primary matter is placed first. It is absurd, in Calvin’s view, to talk about righteousness or justice (coram Deo) while ignoring its chief part, recognition and worship of the true God. Indeed, “[i]t is vain,” he says, “to talk of righteousness apart from religion,” as though we could sanitize the moral or natural law by dislodging its “first foundation” and amputating its capital point of offense–“capital,” that is, as derived from caput, “head”; for to separate (n.b., not “distinguish”) the two halves of the Law in that way would be to sever the Law’s head from its body and to leave it lying corpsewise on the ground like Priam or Ozymandias–more than that, it would be as the evacuation of the soul that animates the entire organism.1
It will now be proper to consider what is meant by the division of the divine Law into Two Tables. It will be judged by all men of sense from the formal manner in which these are sometimes mentioned, that it has not been done at random, or without reason. Indeed, the reason is so obvious as not to allow us to remain in doubt with regard to it. God thus divided his Law into two parts, containing a complete rule of righteousness, that he might assign the first place to the duties of religion which relate especially to His worship, and the second to the duties of charity which have respect to man. The first foundation of righteousness undoubtedly is the worship of God. When it is subverted, all the other parts of righteousness, like a building rent asunder, and in ruins, are racked and scattered. What kind of righteousness do you call it, not to commit theft and rapine, if you, in the meantime, with impious sacrilege, rob God of his glory? or not to defile your body with fornication, if you profane his holy name with blasphemy? or not to take away the life of man, if you strive to cut off and destroy the remembrance of God? It is vain, therefore, to talk of righteousness apart from religion. Such righteousness has no more beauty than the trunk of a body deprived of its head. Nor is religion the principal part merely: it is the very soul by which the whole lives and breathes. Without the fear of God, men do not even observe justice and charity among themselves. We say, then, that the worship of God is the beginning and foundation of righteousness; and that wherever it is wanting, any degree of equity, or continence, or temperance, existing among men themselves, is empty and frivolous in the sight of God. We call it the source and soul of righteousness, in as much as men learn to live together temperately, and without injury, when they revere God as the judge of right and wrong. In the First Table, accordingly, he teaches us how to cultivate piety, and the proper duties of religion in which his worship consists; in the second, he shows how, in the fear of his name, we are to conduct ourselves towards our fellow-men. Hence, as related by the Evangelists (Mt. 22:37; Luke 10:27), our Saviour summed up the whole Law in two heads—viz. to love the Lord with all our heart, with all our soul, and with all our strength, and our neighbour as ourselves. You see how, of the two parts under which he comprehends the whole Law, he devotes the one to God, and assigns the other to mankind.