If they do not persevere. Romans 11:19-23 You will say then, "Branches were broken off so that I might be grafted in." Quite right, they were broken off for their unbelief, but you stand by your faith. Do not be conceited, but fear; for if God did not spare the natural branches, He will not spare you, either. Behold then the kindness and severity of God; to those who fell, severity, but to you, God's kindness, if you continue in His kindness; otherwise you also will be cut off. And they also, if they do not continue in their unbelief, will be grafted in, for God is able to graft them in again. Hebrews 10:23-26 Let us hold unswervingly to the hope we profess, for he who promised is faithful. If we deliberately keep on sinning after we have received the knowledge of the truth, no sacrifice for sins is left, how much more severely do you think a man deserves to be punished who has trampled the Son of God under foot, who has treated as an unholy thing the blood of the covenant that sanctified him, and who has insulted the Spirit of grace? Ez. 18:24 But if a righteous man turns from his righteousness and commits sin and does the same detestable things the wicked man does, will he live? None of the righteous things he has done will be remembered. Because of the unfaithfulness he is guilty of and because of the sins he has committed, he will die.

NEVER. It's not possible.

If you can't be good enough to earn your salvation, then you can't be bad enough to lose it. Jesus' sacrifice was strong enough and good enough to take care of our sin. People who teach we can loose our salvation are saying Jesus wasn't good enough and the person who sins is stronger than Jesus.