I am going to lay out two points for this discussion. One, I’m going to assume that by “salvation” we are talking about heavenly salvation. Two, only God really knows who would fall in that category for sure because he is the only judge of it. That being stated:
While Jesus was on the earth he made a comment which is appropriate to the subject when his miracles were attributed to Beelzebub:
“So I tell you, every sin and blasphemy will be forgiven, but blasphemy against the Spirit will not be forgiven. Whoever speaks a word against the Son of Man will be forgiven, but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the one to come."
(Mat 12:31-32; ISV)
So it is possible to fall away and commit acts for which there is no forgiveness. Becoming and enemy of the Way and speaking against it can bring one perilously close to what Jesus was talking about since the True Church has God’s Holy Spirit and the works they perform are through its influence. We find confirmation of that thought in Paul’s letter to the Hebrews where he states:
“For if we choose to go on sinning after we have learned the full truth, there no longer remains a sacrifice for sins, but only a terrifying prospect of judgment and a raging fire that will consume the enemies of God. Anyone who violates the Law of Moses dies without mercy ‘on the testimony of two or three witnesses.’ How much more severe a punishment do you think that person deserves who tramples on God's Son, treats as common the blood of the covenant by which it was sanctified, and insults the Spirit of grace? For we know the one who said, ‘Vengeance belongs to me; I will pay them back,’ and again, ‘The Lord will judge his people.’ It is a terrifying thing to fall into the hands of the living God!”
(Heb 10:26-31; ISV)
Paul states clearly that once one has “learned the full truth” and presumably joined the true church in their calling then chooses to sin, then there is no sacrifice left for them and such sin is no longer covered. Since he included the thought of “tramples on God’s Son, treats as common the blood of the covenant by which it was sanctified,” it would seem that such sin would not only be deliberate but of a similar nature as that which Jesus warned against. Basically, one can’t accidentally fall into this sin; they have to go out of their way to enter that category.
To sum it up, the belief of some that once one is “saved” they cannot be lost doesn’t really hold up. Jesus himself warned that there was a sin which could earn one eternal unforgiveness, blasphemy of the Holy Spirit. Paul enlarged upon the idea and his words make it clear he wrote about those who were “saved,” that is part of the true Church called to heaven. Paul makes it clear in no uncertain terms that those who willfully fall away and sin will fall into God’s hand of judgment and be lost.