What about "falling away"?
Hebrews 6:4-6. It is impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit, who have tasted the goodness of the word of God and the powers of the coming age, if they fall away, to be brought back to repentance, because to their loss they are crucifying the Son of God all over again and subjecting him to public disgrace.
Hebrews 10:26-31. If we deliberately keep on sinning after we have received the knowledge of the truth, no sacrifice for sins is left, but only a fearful expectation of judgment and of raging fire that will consume the enemies of God. Anyone who rejected the law of Moses died without mercy on the testimony of two or three witnesses. 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? For we know him who said, "It is mine to avenge; I will repay," and again, "The Lord will judge his people." It is a dreadful thing to fall into the hands of the living God.
If all are reconciled to God, why these warnings about "falling away"?
The gospel proclaims that God's love never ceases, and Christ's love in us, ministered by the Spirit, for the Father and for all humanity never ceases (the picture from the parable of the prodigal son above is a powerful illustration).
We can participate in the fellowship in which God has included us in Christ, or we can resist it (God grants us freedom to choose). But either way, the relationship that includes us exists in Christ between God and all humanity. The stunning truth is that God, in grace, has "accepted" us "in the beloved" (Ephesians 1:6, KJV). In that sense, all humanity is elect and saved in Christ ("the beloved" of the Father). It is not therefore a question of losing one's salvation, but a question of participating in the salvation we already have. If we don't participate in it, it is akin to not having it, and in that spirit, we cannot enjoy the fruit of it. But we do have it, whether we participate in it or not.
It helps to stop thinking of salvation as a sort of moment-in-time reward that can be won and lost, and start thinking of salvation as being in loving fellowship with the God who made us, sustains us, loves us and dwells in us.
In other words, salvation isn't something we "have," per se; it is something we participate in. Maybe we could compare it to "having" electricity, but to have it is meaningless unless you plug in a light or an appliance and actually use the electricity. Not the best analogy, but I trust you see what I mean.
To not remain in Christ is to not participate with Christ through the Spirit in the Father. That participation is what salvation is. So to not participate in it is like not having it. We participate in faith, trusting Christ to be for us who he says he is for us. The Spirit works in us, transforming us into the image of Christ. The Spirit also works to bring us back when we stray, because God is always faithful in his love for us even when we are not faithful. So from God's side, Christ came into the world not to condemn the world, but to save it, and that is what he did—save it.
From our side, we can trust him and participate in that salvation, or we can refuse to trust him and ignore or resist that salvation, which means to ignore or resist our fellowship with him. Either way, God saved us in Christ, but only if we embrace that grace can we participate in the joy of knowing God - our experience of his salvation that is assured and thus secure in Christ.
The above mentioned passages in Hebrews 6 and 10 warn Christians (believers) who are participating in this salvation in Jesus, not to turn away ("fall away") from that participation in unbelief. To do so has serious consequences. The strong admonition,therefore, is to "hold unswervingly to the hope we profess, for he who promised is faithful” (10:23). The goal of the author is to spur believers on to greater participation through love and faith (v24). He is optimistic about their response, for he says that these people “are not of those who shrink back and are destroyed, but of those who believe and are saved” (v39).
The Bible affirms that all who participate, through faith, in the salvation that is theirs in Jesus Christ are "safe" (saved), and nothing "can snatch them out of his hand.” The gospel emphasizes the infinite faithfulness of Jesus Christ, and his total sufficiency for our salvation. Thus there is every reason for us to have a sense of assurance, and every reason to confidently invite non-believers to receive (subjectively/personally) what is already theirs (objectively/universally) in Jesus.
We need not worry about our salvation, asking, “What if I fail? " The fact of the matter is that already we have failed. Jesus is the one who saves us, and he doesn't fail. Can we fail to accept him? Yes, but even here we can trust him to do his work - to share with us his faith (see Galatians 2:20, KJV). Trusting in Jesus, we have joy, not fear. We have peace, not anxiety.