My answer follows:
Your question relates to the doctrine of "the perseverance of the saints." The OPC believes in the perseverance of the saints, which teaches that true saving faith perseveres unto eternal life. A person who has received eternal life does not lose it. In this regard, the OPC accepts the Westminster Confession of Faith (http://opc.org/wcf.html) as a faithful summary of what the Scripture teach on this important doctrine. Indeed an entire chapter is devoted to this very matter as follows:
Of the Perseverance of the Saints
1. They, whom God hath accepted in his Beloved, effectually called, and sanctified by his Spirit, can neither totally nor finally fall away from the state of grace, but shall certainly persevere therein to the end, and be eternally saved.
2. This perseverance of the saints depends not upon their own free will, but upon the immutability of the decree of election, flowing from the free and unchangeable love of God the Father; upon the efficacy of the merit and intercession of Jesus Christ, the abiding of the Spirit, and of the seed of God within them, and the nature of the covenant of grace: from all which ariseth also the certainty and infallibility thereof.
3. Nevertheless, they may, through the temptations of Satan and of the world, the prevalency of corruption remaining in them, and the neglect of the means of their preservation, fall into grievous sins; and, for a time, continue therein: whereby they incur God's displeasure, and grieve his Holy Spirit, come to be deprived of some measure of their graces and comforts, have their hearts hardened, and their consciences wounded; hurt and scandalize others, and bring temporal judgments upon themselves.
In scriptural terms, we can think of the prayer that Jesus says He prayed for Peter, when Satan sought to ‘sift him like wheat’ (Luke 22:31). Jesus says, however, “but I have prayed for you, that your faith may not fail; and you, when once you have turned again, strengthen your brothers” (Luke 22:32). Jesus obviously knows that Peter’s faith will not fail, for He knows that Peter shall turn again, even after his sorrowful three-fold denial, to prove himself as a faithful apostle. Yes, true Christians sometimes stumble, even fall into grievous sins; but they shall not finally and eternally fall away.
True faith, saving faith, is a gift from God (Ephesians 2:8). The Westminster Shorter Catechism (http://opc.org/sc.html) says, “Faith in Jesus Christ is a saving grace, whereby we receive and rest upon him alone for salvation, as he is offered to us in the gospel” (Answer to Question 86). He who believes in Christ has eternal life (John 6:47). Jesus will lose none of those whom the Father has given to Him (compare John 6:37 with John 6:39). No one can snatch the true sheep of the Good Shepherd out of His hand (John 10:28). “My sheep hear My voice, and I know them, and they follow Me; and I give eternal life to them, and they shall never perish” (John 10:27-28). True believers “are protected by the power of God through faith for a salvation ready to be revealed in the last time” (I Peter 1:5).
Still, we must recognize that not all who profess faith necessarily have true saving faith. The true, God-given faith perseveres to the end. But in the gospel of John, we read of folks who ‘believed’ in Christ, who eventually turned away from Him. That is, they believed He was a good teacher. They believed He could do miracles. They even believed to the extent that they wanted to make Him their earthly king John 6:15). But we read of some of those very followers of Christ (‘disciples’) who “withdrew, and were not walking with Him anymore” (John 6:66). They withdrew because they could not accept Jesus as the ‘true food’…‘the bread that came down out of heaven’ (John 6:53-58). Amazingly, this was just a short time after they had witnessed the feeding of the 5000 that they stopped following Him. And it is in that same context that Jesus speaks to the twelve, saying that He had chosen them, yet He knew that one of them was a devil (John 6:70).
Jesus knew whom He had chosen; He knew one would betray Him. We do not know God’s secret decrees, “For Jesus knew from the beginning who they were who did not believe, and who it was that would betray Him” (John 6:64). We cannot look into men’s hearts as Jesus the Son of God could. So it is that wolves sometimes disguise themselves as sheep and make their way into the church. Members are received into the church on their testimony and the credibility of their walk in keeping with those words. Elders should exercise great care in receiving members, but even so, they cannot be infallibly sure that true saving faith is at work in a person’s heart. Sometimes, those who seem to have faith, fall away and are lost, proving that their ‘faith’ was not real saving faith after all and that they were never really saved.
This is a most sobering and humbling truth. May God’s grace comfort and encourage you according to that true faith that looks to Jesus Christ alone for salvation from sin and Satan and death.
To God be the glory,
R. Daniel Knox, Pastor
Grace Orthodox Presbyterian Church