When an alleged Christian is hardened in the bad way, its because he is a child of the Devil, a Tare sown among the wheat.
You cannot lose your salvation if you are a child of God, nor would you live like a Tare. That's why "we can know them by their fruits":
16 "You will know them by their fruits. Do men gather grapes from thornbushes or figs from thistles?
17 "Even so, every good tree bears good fruit, but a bad tree bears bad fruit.
18 "A good tree cannot bear bad fruit, nor can a bad tree bear good fruit.
19 "Every tree that does not bear good fruit is cut down and thrown into the fire.
20 "Therefore by their fruits you will know them. (Matt. 7:16-20 NKJ)
Once we are in God's hands, no one (which includes yourself) can take you out. Eternal life isn't eternal if you can perish after you get it:
27 "My sheep hear My voice, and I know them, and they follow Me.
28 "And I give them eternal life, and they shall never perish; neither shall anyone snatch them out of My hand.
29 "My Father, who has given them to Me, is greater than all; and no one is able to snatch them out of My Father's hand. (Jn. 10:27-29 NKJ)
When an alleged Christian is hardened in the bad way, its because he is a child of the Devil, a Tare sown among the wheat.
You cannot lose your salvation if you are a child of God, nor would you live like a Tare. That's why "we can know them by their fruits":
16 "You will know them by their fruits. Do men gather grapes from thornbushes or figs from thistles?
17 "Even so, every good tree bears good fruit, but a bad tree bears bad fruit.
18 "A good tree cannot bear bad fruit, nor can a bad tree bear good fruit.
19 "Every tree that does not bear good fruit is cut down and thrown into the fire.
20 "Therefore by their fruits you will know them. (Matt. 7:16-20 NKJ)
The problem with this theory is that you discard a believer's free will by saying that the person can never fall away. Unlike this theory, Peter gave the ingredients to make one's calling and election sure.
2 Peter 1
[4] Whereby are given unto us exceeding great and precious promises: that by these ye might be partakers of the divine nature, having escaped the corruption that is in the world through lust.
[5] And beside this, giving all diligence, add to your faith virtue; and to virtue knowledge;
[6] And to knowledge temperance; and to temperance patience; and to patience godliness;
[7] And to godliness brotherly kindness; and to brotherly kindness charity.
[8] For if these things be in you, and abound, they make you that ye shall neither be barren nor unfruitful in the knowledge of our Lord Jesus Christ.
[9] But he that lacketh these things is blind, and cannot see afar off, and hath forgotten that he was purged from his old sins.
[10] Wherefore the rather, brethren, give diligence to make your calling and election sure: for if ye do these things, ye shall never fall:
[11] For so an entrance shall be ministered unto you abundantly into the everlasting kingdom of our Lord and Saviour Jesus Christ.
Once we are in God's hands, no one (which includes yourself) can take you out. Eternal life isn't eternal if you can perish after you get it:
27 "My sheep hear My voice, and I know them, and they follow Me.
28 "And I give them eternal life, and they shall never perish; neither shall anyone snatch them out of My hand.
29 "My Father, who has given them to Me, is greater than all; and no one is able to snatch them out of My Father's hand. (Jn. 10:27-29 NKJ)
Just as with any gift in life, it can be neglected, misused, not cared for and eventually lost or stolen.