John says things as they are. He knew that the "whole world" for a period of time was "under the control of the evil one" (v.19). He also knew that "the evil one" can't "touch" the believer who puts his trust in the "One [Christ] who was born of God" (v.lSb). He knew that one sin could separate man from God eternally (the sin of unbelief — 16c), and only one act could forever unite man with God (belief "in the name of the Son of God" — v.13). And, he knew that if the believer was in tune with God's will on a matter, God would respond to an obedient petition (v.14).
But there was one thing John knew that rose above all else — in fact he calls it his "testimony": "God has given us eternal life, and this life is in His Son" (v.ll). John knew that God was utterly holy and could not tolerate sin. He also knew that God was just and had to punish the sinner. And, most importantly in John's thinking, he knew that God was love, hating sin but loving the sinner. So what was God to do? The "wages of sin is death", but "He is not willing that any should perish."
John knew that God sent Christ to take the penalty of our sin upon Himself — "he is the atoning sacrifice for our whole world" (2:2). Jesus is the key to life. If you have Him you have life — "He is the true God and eternal life" (5:20c).