>> They do alway err in their heart; and they have not known my ways.<<
>> Take heed, brethren, lest there be in any of you an evil heart of unbelief,<<
Those that fall away have not known my ways, and had an evil heart of unbelief to begin with. Also because of unbelief they were broken off.
Paul said Believe on the Lord Jesus Christ and you will be saved. Scripture clearly teaches that those you would say lost their salvation never had it to begin with because they had an evil heart of unbelief, they have not known my ways, and because of unbelief they were broken off. So your contention that they lost their salvation is in error.