Matthew 13:
18"Listen then to what the parable of the sower means: 19When anyone hears the message about the kingdom and does not understand it, the evil one comes and snatches away what was sown in his heart. This is the seed sown along the path. 20The one who received the seed that fell on rocky places is the man who hears the word and at once receives it with joy. 21But since he has no root, he lasts only a short time. When trouble or persecution comes because of the word, he quickly falls away. 22The one who received the seed that fell among the thorns is the man who hears the word, but the worries of this life and the deceitfulness of wealth choke it, making it unfruitful. 23But the one who received the seed that fell on good soil is the man who hears the word and understands it. He produces a crop, yielding a hundred, sixty or thirty times what was sown."
1 John 2:19
They went out from us, but they did not really belong to us. For if they had belonged to us, they would have remained with us; but their going showed that none of them belonged to us.
How can you lose salvation? You can believe you had something you really didn't have in the first place.
2 Peter 2:
17These men are springs without water and mists driven by a storm. Blackest darkness is reserved for them. 18For they mouth empty, boastful words and, by appealing to the lustful desires of sinful human nature, they entice people who are just escaping from those who live in error. 19They promise them freedom, while they themselves are slaves of depravity—for a man is a slave to whatever has mastered him. 20If they have escaped the corruption of the world by knowing our Lord and Savior Jesus Christ and are again entangled in it and overcome, they are worse off at the end than they were at the beginning. 21It would have been better for them not to have known the way of righteousness, than to have known it and then to turn their backs on the sacred command that was passed on to them. 22Of them the proverbs are true: "A dog returns to its vomit,"[Prov. 26:11 ]and, "A sow that is washed goes back to her wallowing in the mud."