In answering, Trinitarian theology begins with the all-important question, Who is Jesus? The Bible answers that Jesus is the permanent union of God with all humanity. On the basis of this truth (this personal union) we can answer our immediate question: Can one lose one's salvation?
Unfortunately, salvation often is viewed as a transaction with God -- a sort of bookkeeping exercise in which we give God our faith, and in exchange he grants us forgiveness and justification (salvation). But what the nature of the incarnate, crucified, risen and ascended Lord Jesus Christ tells us is that salvation is a relationship -- it's about fellowship not transaction. Moreover, we are shown that this relationship-fellowship between God and humankind has been established and occurs in and through Jesus himself as the permanent union of God and all humanity.
On this basis, our question becomes this: Can one lose one's relationship (fellowship) with God? Dr. Mike Feazell (formerly GCI vice president) answers in a letter. Here is an excerpt:
Salvation is about being in fellowship with God. God's love never ceases, and Christ's love in us, ministered by the Spirit, for the Father and for others never ceases. We can participate in that fellowship in which God has included us in Christ, or we can resist it. However, the relationship exists whether we embrace it or resist it. In that sense, all humanity is elect in Christ and saved in him. It isn't 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 way, not enjoying its fruit. But we do have it, whether we participate in it or not.
It helps when we 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. This is not a perfect analogy, but perhaps it is helpful.
To not remain in Christ is thus not to participate with Christ through the Spirit in the Father. This participation is what salvation is. So not to 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. As we trust, the Spirit works in us, transforming us into the image of Christ. The Spirit also works to bring us back when we don't trust, 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 (John 3:16-17), and that is what he did—save it. From our side, we can trust him and participate in that salvation, or we can not 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 the Father, Son and Spirit.