Herman Witsius, The economy of the covenants, volume 1, pp. 370:
The covenant of grace, or the gospel, strictly so called, which is the model of that covenant, since it consists in mere promises, prescribes nothing properly as duty, requires nothing, commands nothing; not even this, Believe, trust, hope in the Lord, and the like. But it declares, sets forth, and signifies to us, what God promises in Christ, what he would have done, and what he is about to do. All prescription of duty belongs to the law, as, after others, the venerable Voetius has very well inculcated. And we are by all means, to maintain this, if, with the whole body of the reformed, we would constantly defend the perfection of the law, which comprehends all virtues, and all the duties of holiness.