The idea is simple. No one who has believed in Christ can ever fall so far that the hand of God will not lift them up. Jesus put it this way: "All that the Father gives me will come to me, and whoever comes to me I will never drive away. For I have come down from heaven not to do my will but to do the will of him who sent me. And this is the will of him who sent me, that I shall lose none of all that he has given me, but raise them up at the last day" (John 6:37-39).
Theologians call this “the perseverance of the saints” or sometimes “eternal security.” It’s the doctrine we sometimes refer to as “once saved, always saved.” The Westminster Confession of Faith says, "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."
That doesn’t mean we’re immune from sin or get a pass on paying a temporal penalty for sins. The Confession goes on, "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."
You may think that because you failed to manage your household well, battle an ongoing marital struggle, or have committed some truly wicked sin that you have disqualified yourself. That’s simply not true. No matter what you’ve done, you can be forgiven. Without the doctrine of "once saved, always saved" heaven would be empty. (from How God Makes Men)