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Is it guaranteed, or Not?
(Its not!)
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You are rejecting the Word in Ephesians when it says the believer is sealed by the Holy Spirit. You are rejecting what Jesus said in John 5:24
Truly, truly I say to you, he who hears my Word, and believes Him who sent Me, has eternal life, and does not come into judgment, but has passed out of death into life.
If you are relying upon works, as you admit you must be if the death of Christ is insufficient for your sins, then I ask:
How many of these good works do you have to do?
How do you know you've done the right ones?
How do you know you did them correctly?