VIII. Once more, 4thly, It was entirely agreeable, that God should promise Adam by covenant something greater and better, to be obtained after finishing his course of obedience, than what he was already possessed of. What kind of covenant would it have been, to have added no reward to his obedience, and his faithful compliance with the conditions of the covenant, but only a continuation of those blessings which he actually enjoyed already, and which it was not becoming God to refuse to man, whom he had created? Now, Adam enjoyed in Paradise all imaginable natural and animal happiness, as it is called. A greater, therefore, and a more exalted felicity still awaited him; in the fruition of which, he would most plainly see, that “in keeping the divine commands, there is עקב רב, μισθαποδοσιαν μεγαλην, great reward,” Ps. 19:11. Let none object the case of the angels, to whom, he may pretend, nothing was promised by God, but the continuance of that happy state in which they were created. We are here to keep to the apostle’s advice, Col. 2:18. “not to intrude into those things we have not seen.” Who shall declare unto us those things which are not revealed concerning the angels? But, if we may form probable conjectures, it appears to me very likely, that some superior degree of happiness was conferred on the angels, after they were actually confirmed, and something more excellent than that in which they were at first created: as the joy of the angels received a considerable addition, upon beholding the divine perfections, so resplendent in the illustrious work of redemption; and at the consummation of all things, the happiness of all the elect, both angels and men, will be complete; when Christ’s whole body shall appear glorious, and God be glorified and admired in all his saints.
Witsius- Economy of the Covenants, pp. 75-6, Vol. 1