Even a lot of believers who say they believe in Eternal Security, really simply mean by that the Fifth Point of Calvinism, or some variation there of. That certain Sins, or Apostasy, or Bad Doctrine can be used as proof of a lack of true Salvation in a professing Christian.
Like people who believe Salvation can be Lost or in Works Salvation they will miss use verses there for the purposes of showing how impossible it is to earn Salvation.
But they will add to those verses, verses that are about Works as evidence of Faith. The Evidence of Faith in question isn't for other Believers, but about Witnessing. The great Commission actually says in the Greek to "Be A Witness". We should be credible representatives of God's Kingdom. That is why Jesus told the Apostles that The World will Know them by their Love for one another. And why Nathan told David he gave Occasion to the Enemies of The LORD to Blaspheme after he sinned with Bathsheba and killed Uria.
Faith without Works is Dead. But you can't have Dead Faith if you never had Faith.
1 Corinthians 3:15 says "If any man's work shall be burned, he shall suffer loss: but he himself shall be saved; yet so as by fire."
Saul we know was Saved. His Salvation was depicted before he became King. Yet he backslid horribly and became a Villain of David's Story. But when his Death was foretold Samuel said he and his Sons would be with him. I know in the Old Testament, before The Cross all the Saved went to Sheol. But in Luke 16 we're informed it has two distinct compartments for the Saved and Unsaved. What Samuel said there isn't with him as in their both in the same place the same way someone in Racine or Milwaukee are both in Wisconsin. It means they will be together.
Solomon also didn't Persevere to the end. And Lot and Samson were both Worldly. The latter each have New Testament verses confirming their Salvation.
Hebrew 6, and latter parts of Hebrew elaborating on the same Subject are controversial. I agree with Chris White that the Sin in mind here is Apostasy (not bad doctrine, but renouncing your faith). But he's wrong in taking the view that Apostasy is the one exception to Eternal Security.
I highly Recommend what Chuck Missler says on Hebrews 6, (he falls into the same traps Calvinists do on some other verses, but on Hebrew 6 he nails it). He points out issues with the Greek Grammar that aren't easily carried over into English. But mostly bases his point on going back to the Old Testament passage it draws on, Numbers 13-14, the Spies who rejected The Land. God was very angry with them, but Moses Prayed for them and they were Forgiven, BUT they were not allowed to see The Land and that's why they wondered for 40 years. Likewise when a Saved person commits Apostasy Jesus Prays for him just as he Prayed for Peter.
The Prodigal Son squandered his Inheritance but did not lose his Sonship. When Jesus in Matthew refers to the "Outer Darkness" that is not Hell. That refers to being Outside New Jerusalem, where The Light of God will be. Some people refuse to admit that not all the Saved are New Jerusalem. But Revelation 21:24-27 makes clear there are entire nations outside. Many are simply because they weren't Saved during the Church Age, but there will also be Church Age believers who lose their Citizenship of The Holy City, but not their Salvation.
A certain pastor I don't want to name, who teaches Eternal Security, and preaches against all Five points of Calvinism, still loves to question people's Salvation based on Doctrine.
He says based on "My sheep hear my voice and follow me" that whatever problems they have, a truly Saved person will respond to God's Word. This verse is about Salvation, and another one Calvinists might miss use to support their view of Predestination, but what their wrong on is the cause and effect.
This same Pastor agrees with me on using Saul to Prove Eternal Security, and also Solomon. But both of them heard God's Word from anointed Prophets and Priests trying to turn them from their backslidden paths yet they did not do so.
The KJV Biblical Word most Calvinists I've seen view as what they mean by "Persevere" is Overcome. Mainly how it's used in 1 John and Revelation 2-3. Overcome is used in many senses of many things in The Bible, plenty no Clavinsits thinks is about Salvation.
1 John 5:4-5 says
"For whatsoever is born of God overcometh the world: and this is the victory that overcometh the world, even our faith. Who is he that overcometh the world, but he that believeth that Jesus is the Son of God?"What John means here is clearly all the Saved. And doesn't matter if you Persevered to the end or not, simply Believing means you overcome the world.
Some of the usages in Revelation 2-3 and 21:7 may or may not be using it specially of those who who don't lose their Inheritance. I believe all Seven Churches, even the two worst ones, were congregations of Saved individuals, or else Jesus wouldn't have written to them at all.
You're saved if you Overcome, but not every saved person will Overcome to the end.
Romans 10 says "All who call upon the name of The Lord shall be Saved" no qualifying statements. 1 John 4:2 says " Hereby know ye the Spirit of God: Every spirit that confesseth that Jesus Christ is come in the flesh is of God". And verse 15 says "Whosoever shall confess that Jesus is the Son of God, God dwelleth in him, and he in God."
I'm not saying there are no professing Christians who aren't saved. But those that are are people who were born in the faith, or who converted for the wrong reasons. But everyone who understands they need a Savior and accept Jesus is Saved.
"But to him that worketh not, but believeth on him that justifieth the ungodly, his faith is counted for righteousness." - Romans 4:5
"Therefore we conclude that a man is justified by faith without the
"For by grace are ye saved through faith; and that not of yourselves: it is the gift of God: Not of works, lest any man should boast." - Ephesians 2:8,9