5:13. These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God. (Epistle of St. John the Theologian).
Commentary by St. Justin Popovich:
The most important thing is to know that believing in Christ means to have eternal life. This is the central and preeminent realization of every Christian. I believe and that is why I have eternal life. I have eternal life because I believe. From this realization, fearlessness and confession of faith are born; and readiness, as well, for every sacrifice for Christ, patience and joyous suffering for Christ. This realization becomes not just a guide, but also a supreme, guiding, and moral principle for a Christian. He then normalizes and regulates everything according to it. He then feels and knows that his highest concern is to achieve eternal life through every feeling, every thought, every act, and every deed. Then, this begins for us an unceasing struggle, and unceasing experience of the evangelic virtues, unceasing rebirth of oneself into something newer and newer, into a better and better man; always looking unto the Founder and Finisher of our faith, the Lord Jesus and His labourers and followers (cf. Heb. 12: 1-2). Then, man always keeps in mind the evangelic batter call and rule: Fight the good fight of faith, lay hold on eternal life, whereunto thou art also called (I Tim. 6:12). You are a Christian; your calling is to achieve eternal life, to fight day and night for eternal life; and for the safe of it, you must bear every torment, every hardship, every struggle with joy. For our light affliction, which is but for a moment, worketh for us a far more exceeding and eternal weight of glory (II Cor. 4:17), to us who look at the eternal and search for that which is eternal; for our main purpose is eternal life (cf. II Cor. 4:18).
Taken from the book: Commentary on the Epistles of St. John the Theologian, by St. Justin Popovich