Things are not always that simple when doing theology. Regardless of which theological system you subscribe to, there is [always] a verse or passage that seems to oppose or contradict your system. As faithful students of Scriptures, we must not hide verses that go against our system and pretend that they don’t exist, but rather, we must work out the nuances and do our best to reconcile them. It is abundantly clear that salvation can not be lost, but what do we do with warning passages, and verses that seem to teach otherwise? On the other hand, those that oppose the Eternal Security of true believers will be quick to point out all the warning passages, but then conveniently ignore the faithfulness of God and His promises to preserve those that are His. A lot can be said on this topic, but the point here is that we must do our best to reconcile them (for and against passages), and to give an answer for why we arrived at such a conclusion after having looked at both the [for and against].
We who know better should not be quick to condemn other Christians. One can teach truth that is incomplete, and if so, take him aside (Acts 18:24-28) and explain the way of God more accurately and fully.