That this belief is contrary to the Bible & constant Christian teaching.
Recall this Scripture: "If we have died with him [in baptism; see Rom. 6:3-4] we shall also live with him; if we persevere we shall also reign with him" (2 Tim. 2:11-12). So, ff we do NOT persevere, we shall NOT reign with him. In other words, Christians can forfeit heaven. Jesus tells us, "He who endures to the end will be saved" (Matt. 24:13; cf. 25:31-46). Ergo, if you do not endure to the end, well........
The Bible makes it clear that Christians have a moral assurance of salvation (God will be true to his word and will grant salvation to those who have faith in Christ and are obedient to him [1 John 3:19-24]), but the Bible does NOT teach that Christians have a guarantee of heaven. There can be no absolute assurance of salvation. The Bible says, "See, then, the kindness and severity of God: severity toward those who fell, but God's kindness to you, provided you remain in his kindness, otherwise you too will be cut off" (Rom. 11:22-23; Matt. 18:21-35, 1 Cor. 15:1-2, 2 Pet. 2:20-21).
Note that this includes an important condition: "provided you remain in his kindness." It is saying that Christians can lose their salvation by throwing it away. He warns, "Whoever thinks he is standing secure should take care not to fall" (1 Cor. 10:11-12). , Paul admitted that even he could fall away: "I pummel my body and subdue it, lest after preaching to others I myself should be disqualified" (1 Cor. 9:27). In saying this he points out that even he cannot be infallibly sure of his own present state or of his future salvation.