This is a point that needs to be emphasized here. I've seen a page & a half of opposition without any scripture to support it. Everyone should ensure to be adhering to the Rules of the A&T, particularly in using scripture to refute anything someone has said.
When the issue being discussed is one that has divided the Christian community approximately in half, I'm not sure what is accomplished by posters slinging Bible verses back and forth. Christian theologians, scholars and pastors of the highest level have found the OS-Not-AS position in the Bible since the earliest days of Christianity and continuing until the present time - OSAS is a comparative newcomer, in fact. Since I am apparently obligated to sling Bible verses, I will note that these OS-Not-AS theologians, scholars and pastors have found the doctrine in Hebrews 6:4-6, Hebrews 10:26-27, 1 Corinthians 15:1-2, Colossians 1:21-23, Matthew 10:22, John 15:1-8 and many others. No one on this forum is going to "prove" these theologians, scholars and pastors "wrong." The best someone holding the OSAS position can do is show that OSAS likewise has biblical support and is not unreasonable. (Hebrews 10:14 doesn't strike me as one of the stronger verses in support of OSAS; it strikes me as equally compatible with OS-Not-AS, but whatever - neither position hangs on a single verse.)
The complaint that I and others are suggesting the Bible is "contradictory" or "teaches untruth" is a red herring. The fact that Bible verses appear to point in different directions does not mean the Bible contradicts itself or teaches untruth. I've said on other posts that it is indeed somewhat troubling to me that the Bible isn't clearer about certain key doctrines, but the fact is it isn't. Requiring us to think, interpret, debate and pray for guidance is apparently part of God's plan.
The debate over this doctrine brings to mind Pascal's Wager: If I bet OS-Not-AS is true and live my life as though it were, what have I lost? If I bet OSAS is true and live my life as though it were, however, I could be making a tragic mistake. This is one of the key arguments against the OSAS position - i.e., it teaches a cheap and easy salvation that can easily be misinterpreted as suggesting "It doesn't matter what I think or do from now on." This argument would have no force, of course, if OSAS were clearly and unequivocally taught in the Bible - but it isn't.
In terms of harmonizing the verses, it seems to me that the verses pointing toward OSAS can
far more easily be reconciled with the OS-Not-AS verses than the OS-Not-AS verses can be harmonized with the OSAS verses. The OS-Not-AS verses are pretty much impossible to reconcile with the OSAS verses, except through unconvincing semantic gyrations. The OSAS verses, on the other hand, can largely be read as a "subset" of or "shorthand" for OS-Not-AS. This doesn't mean OS-Not-AS is "right," but it is a point in its favor.
Those who hold to the OSAS position: Good for you. I hope for your sake it is true. It has a definite appeal. It has biblical support. It has been held by many theologians, scholars and pastors. It was my understanding when I was born again 45 years ago. It doesn't seem to me the position that has the stronger biblical support, but it could certainly be true. But don't try to prove it is "right" and OS-Not-AS is "wrong" because that effort is going nowhere.
Somewhat oddly, I found this tidy passage on a Baptist site advocating for the OSAS position:
It is important to further emphasize the fact that the doctrine of eternal security does not promise safety for anyone who merely professes Christ. In the following study we see that the Bible connects eternal security only with the true believer, the one who has been born again, and differentiates him with the mere professor. Who has eternal security --
(1) Those who continue in the word (
Jn. 8:31,
32).
(2) Those who follow Christ (
Jn. 10:27-28).
(3) Those who bring forth fruit (
Jn. 15:2;
Lk. 3:9).
(4) Those who are led by the Spirit of God (
Ro. 8:14-15).
(5) Those who have been born again (
2 Co. 5:17;
Ep. 2:10;
Ga. 6:15).
(6) Those who are sanctified from an unrighteous way of life (
1 Co. 6:9-11).
(7) Those who have demonstrated their election (
1 Th. 1:4-10).
(8) Those who depart from iniquity (
2 Ti. 2:19).
(9) Those who maintain their confidence in Christ (
He. 3:14).
(10) Those who have an undivided, convinced faith (
He. 4:10,
11).
(11) Those who evidence the “things that accompany salvation” (
He. 6:9-12).
(12) Those who are looking for Christ’s return (
He. 9:28).
(13) Those who remain patient and steadfast in tribulations (
He. 10:35-39).
(14) Those who are in the truth and continue in the truth (
1 Jn. 2:19-21; 2 Jn. 1-2).
(15) Those who are purifying themselves (
1 Jn. 3:1-3).
(16) Those who love the brethren (
1 Jn. 3:14).
In this instance, OSAS sounds to me an awful lot like OS-Not-AS. If only a "true believer" has "eternal security" and being a true believer equates to "fulfilling conditions (1)-(16) inclusive," how is that different from OS-Not-AS?