The tricky thing about this doctrine is that "Once Saved, Always Saved" is wonderfully true as the Bible teaches it; and a false delusion as taught in many mainline churches.
    In the first case, the Bible plainly says that one’s salvation can’t be lost once it is a reality (Jn.10:27-29; Rom.8:35-39; Eph.1:13,14; 4:30; Heb.9:12; etc.).
    In the second case, many churches teach that eternal salvation is secured simply by public acknowledgement of Christ as savior, repeating a "sinner’s prayer", avowing support for a church, etc. Repentance & works are minimized or ignored.
    The point is this: A person can acknowledge Christ, repeat the prayer, etc. with a heartfelt conviction, being eager to repent, renew his mind, and prayerfully serve the Lord. This person is truly convicted and saved eternally. Another person can do all the above in a perfunctory way for one reason or another, count himself /herself saved because the preacher declares it, and then proceed to live any way he/she wants...disregarding the Scriptures which demand obedience and change and attending to the good works they were ordained to do (Eph.2:10).
    For too long many preachers and churches have built up their numbers and influence based on luring people with this easy believism and grace on demand. They have almost worn out Ephesians 2:8,9 and have inveighed against "works" as being a requirement of salvation. They thus ignore and have been strangely quiet about verse 10 which says plainly they aren’t saved unless they are walking in the good works they were created to perform! This contra-Scriptural "feel good Churchianity" has deceived many into thinking they are saved when they are not, and that all in "their church" are Heaven bound, when--according to I Peter 4:17,18; etc.--maybe one in four will "scarcely" make it.
    When the prepositional phrase "at the house of God" is repeated throughout these verses--as it is grammatically understood to be--a great revelation for churches today leaps out! These verses say that God’s Final Judgement on the world begins at the churches! The buck stops at the churches! It is the churches that God blames for the Devil’s success in the world! It is the churches that are guilty of diluting His Word, allowing it to fit with the world’s false science idol (Copernicanism, Darwinism), and with natural, worldly concepts about what is good and acceptable in politics, finance, sex, art, family, etc., and religion.
    As for those who are merely playing church, and counting themselves good and saved...even though they don't try to be obedient to the Word (v.17), and even if they are ungodly sinners (v.18), God says No Way! He says that even the righteous "at the house of God" will "SCARCELY" be saved (v.18)!!  (HERE)
    Why such Judgement on the churches?! Again, the answer is easy: They have helped promote the Devil’s Pharisaic deception that church rules and teachings --however deeply tainted and compromised with the world’s rules and teachings they may be--override the rules and teachings of God’s Word when the two conflict. Wrong! Click here to order a book with all the Scriptures that spell out the simple truth that Church-based Salvation is Conditional, i.e., that it is based on studying and listening and ever seeking to be Obedient to the Word.