I have already read these articles. J.C. Ryle gives the scriptural answer.

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Reader, you may be sure that Paul was the last man in the world to build his assurance on anything of his own...he looked away from self to Jesus, and was not afraid. He remembered that anchor within the veil, which is both sure and steadfast; — he remembered the word, and work, and constant intercession of Him that loved him and gave Himself for him. And this it was, and nothing else, that enabled him to say so boldly, “A crown is laid up for me, and the Lord shall give it to me”; and to conclude so surely, “The Lord will preserve me: I shall never be confounded.”