Job had assurance
"I know that my Redeemer lives, and that in the end he will stand upon the earth. And after my skin has been destroyed, yet in my flesh I will see God; I myself will see him with my own eyes—I, and not another. How my heart yearns within me" (Job 19:25-27).
The Spirit testifies with our spirit that we are God's children
"The Spirit himself testifies with our spirit that we are God’s children. Now if we are children, then we are heirs—heirs of God and co-heirs with Christ, if indeed we share in his sufferings in order that we may also share in his glory" (Romans 8:16-17).
The Bible assures us that we may know we have eternal life
"I write these things to you who believe in the name of the Son of God so that you may know that you have eternal life" (1 John 5:13).
Paul had assurance of eternal life
"That is why I am suffering as I am. Yet I am not ashamed, because I know whom I have believed, and am convinced that he is able to guard what I have entrusted to him for that day" (2 Timothy 1:12).