The principal duties of love to God above all, and to each other for His sake, from whence all the other duties flow, are so excellent that I cannot imagine any more noble work for the holy angels in their glorious sphere. They are the chief works for which we were at first framed in the image of God, engraven upon man in the first creation, and for which that beautiful image is renewed on us in our new creation and sanctification by Jesus Christ, and shall be perfected in our glorification. They are works which depend not merely on the sovereignty of the will of God, to be commanded or forbidden, or left indifferent, or changed, or abolished at His pleasure, as other works that belong either to the judicial or ceremonial law, or to the means of salvation prescribed by the gospel; but they are, in their own nature, holy, just and good (Rom. 7:12), and suitable for us to perform because of our natural relation to our Creator and fellow creatures; so that they have an inseparable dependence on the holiness of the will of God, and an indispensable establishment thereby. They are works sufficient to render the performers holy in all manner of conversation, by the fruits which they bring forth, if no other duties had ever been commanded; and by which the performance of all other duties is sufficiently established as soon as they are commanded; and without which, there can be no holiness of heart and life imagined; and to which, it was one great honor of Mosaical, and is now of evangelical ordinances, to be subservient for the performance of them, as means which shall cease when their end, this never-failing charity, is perfectly attained (1Cor. 13). They are duties which we were naturally obliged to, by that reason and understanding which God gave to man at His first creation to discern what was just and suitable for him to do, and to which even heathens are still obliged by the light of nature, without any written law, or supernatural revelation (Rom. 2:14, 15).
Therefore they are called natural religion, and the law that requires them is called the natural law and also the moral law; because the manners of all men, infidels as well as Christians, ought to be conformed to it and, if they had been fully comformable, they would not have come short of eternal happiness (Matt. 5:19; Luke 10:27, 28), under the penalty of the wrath of God for the violation of it. This is the true morality which God approves of, consisting in a conformity of all our actions to the moral law. And, if those that, in these days, contend so highly for morality, do understand no other than this, I dare join with them in asserting that the best morally principal man is the greatest saint; and that morality is the principal part of true religion, and the test of all other parts, without which faith is dead and all other religious performances are a vain show and mere hypocrisy: for the faithful and true Witness has testified, concerning the two great moral commandments of love to God and our neighbor, that there is none other commandment greater than these, and that on them ‘hang all the law and the prophets’ (Matt. 22:36-40; Mark 12:31).
Walter Marshall, The Gospel Mystery of Sanctification (Kindle Edition)