Ok, I agree with you at least on the part, we are not God and don't know for sure.
If they were saved, and have renounced Jesus Christ as Messiah and Lord, then they no long believe, and are no longer believers.
Believers are promised eternal life.
A believer is someone who obeys the Gospel.
Those who believe for a while, then return to unbelieving, are not promised eternal life, but eternal death in the lake of fire, since they have no way to be forgiven their sins.
7 He who overcomes shall inherit all things,and I will be his God and he shall be My son. 8 But the cowardly, unbelieving, abominable, murderers, sexually immoral, sorcerers, idolaters, and all liars shall have their part in the lake which burns with fire and brimstone, which is the second death.” Revelation 21:7-8
JLB