There are some who believe that it is possible for Christians to have true faith one day and lose it the next. Not all Christians affirm the perseverance of the saints and the unchangeableness of election. In Reformed theology, however, perseverance of the saints is affirmed. Reformed theology teaches that those who are God’s elect will not fall away because he has a sovereign and gracious hold on them. This teaching is based on various texts and truths of Scripture (e.g. Ps. 37:28, Heb. 13:5, etc.). Here’s how Zacharias Ursinus discussed this topic in his Commentary on the Heidelberg Catechism:
This question has already to a certain extent been answered in what we have said of the unchangeableness of election, and of the perseverance of the saints. The elect when they are once truly in the church of the saints, may indeed sometimes fall, but they never wholly and finally depart from it; not wholly, because they never so fall that they may become the enemies of God and the church; nor yet finally, because they do not continue in apostacy, but do most certainly at length repent and turn to God. “A bruised reed shall he not break, and the smoking flax shall he not quench.” “Neither shall any man pluck them out of my hand.” (Is. 42:3. John 10:28.) But all the reprobate, and hypocrites do at length go out of the church, and with the gifts which they had, they lose also those which they seemed to have. “They went out from us, but they were not of us, for if they had been of us, they would no doubt have continued with us.” (John 2:19.)
Objection: But the saints have also fallen into sin, as David, Peter, etc. Answer: They fall, but not totally, nor finally. Peter fell, but not totally nor finally, for he retained in his heart the love of Christ, although he denied him through fear of danger. He also afterward acknowledged his fall, and wept bitterly over it. Augustine says; “Peter’s faith did not fail in his heart, when he ceased to make confession with his mouth.” Nor did David fall totally; for being reproved of God by his prophet, he did truly repent, and gave evidence that his faith was not wholly lost, but merely slumbered for a time. Hence he prayed, “Take not thy Holy Spirit from me.” Ps. 51:13.) The saints, therefore, never wholly fall. But hypocrites, and the reprobate at length wholly, and finally fall away in such a manner, that they never return to repentance: and because the love of God was never in them, they were never of the member of the elect. Hence it is not to be wondered at, if they at length wholly fall from the church.
From Charlotte, NC