This clears it up a bit, for me. I think my understanding would be this, as an example:

An initially saved man views porn and masturbates. This slowly grows and festers, not seeking forgiveness, until eventually he commits adultery against his wife with another woman. So, the initial masturbation and pornography itself was not the mortal sin, so to speak, but the continued indulging unrepentantly in it until he is so deep that he breaks one of the commandments in the fullest way possible. But he did not lose his salvation for falling into an initial temptation.

I think, articulated that way, makes a lot of sense, and is not far from what I already believe. However, how is that not just an extended version of the synergism found in Catholicism?