God’s immutability is the basis upon which all of our security rests. Because God is unchanging, we can trust his promises. And because it is he who has written our names in heaven, we can be sure that our perseverance is guaranteed.

   It consists not with the majesty of God to call a person effectually to himself today, to make him fit for his eternal love to give him faith, and take away that faith tomorrow. His effectual call is the fruit of his eternal election, and that counsel hath no other foundation but his constant and unchangeable will; a foundation that stands sure, and, therefore, called the foundation of God, and not of the creature; “the foundation of God stands sure, the Lord knows who are his” (2 Tim. ii. 19). It is not founded upon our own natural strength; it may be then subject to changes all the products of nature are. The fallen angels had created grace in their innocency, but lost it by their fall. Were this the foundation of the creature, it might soon be shaken; since man, after his revolt, can ascribe nothing constant to himself, but his own inconsistency. But the foundation is mot in the infirmity of nature, but the strength of grace, and of the grace of God, who is immutable, Who wants not virtue to be able, not kindness to be willing, to preserve his own foundation. To what purpose doth our Saviour tell his disciples their names “were written in heaven” (Luke x. 20), but to mark the infallible certainty of their salvation by an opposition to those things which perish, and have their “names written in the earth” (Jer. xvii 23); or upon the sand, where they may be defaced? And why should Christ order his disciples to rejoice that their names were written in heaven, if God were changeable to blot them out again? or why should an apostle assure us, that though God had rejected the greatest part of the Jews, he had not, therefore, rejected his people elected according to his purpose and immutable counsel; because there are none of the elect of God but will come to salvation? For, saith he, the “election hath obtained it” (Rom. xi. 7); that is, all those that are of the election have obtained it, and the others are hardened. Where the seal of sanctification is stamped, it is a testimony of God’s election and that foundation shall stand sure “the foundation of the Lord stands sure. Having this seal. “The Lord knows who are his;” that is the foundation, the “naming the name of Christ” or believing in Christ, and “departing from iniquity,” is the seal. As it is impossible when God calls those things that are not, but that they should spring up into being and appear before him; so it is impossible but that the seed of God, by his eternal purpose, should be brought to a spiritual life, and that calling cannot be retracted; for that “gift and calling is without repentance” (Rom. xi. 29) and when repentance is removed from God in regard of some works, the immutability of those works is declared; and the reason of that immutability is their pure dependence on the eternal favor and unchangeable grace of God “purposed in himself” (Eph. i. 9, 11) and not upon the mutability of the creature. Hence their happiness is not as patents among men, quam diu bene se gesserint, so long as they behave themselves well; but they have a promise that they shall behave themselves so as never to wholly depart themselves from God (Jer. xxxii. 40): I will make an everlasting covenant with them, that I will not turn away from them to do them good, but I will put my fear into their hearts, that they shall not depart from me.” God will not turn away from them, to do them good, and promiseth that they shall not turn from him forever, or forsake him. And the bottom of it is the everlasting covenant, and therefore, believing and sealing for security are linked together (Eph. i. 13). And when God doth inwardly touch his law, he puts in a will not to depart from it: (Ps. cxix. 102) “I have not departed from Thy judgments;” what is the reason? “For thou hast taught me.”

—Stephen Charnock, The Existence and Attributes of God (Baker Books, 2005), 1:355–356