When John writes his epistle to those who believe in the name of the Son of God to assure them of eternal life (1 John 5:13), he includes awareness of sin. The unbeliever who walks in darkness may not perceive his sin or be pained by it, but the believer will be gripped by his sin and grieve over it with a godly sorrow.
If we say that we have no sin, we deceive ourselves,
and the truth is not in us. (1 John 1:8, NKJV)
Isaiah was never more acutely aware of his sin than when he stood in the presence of the glory of God. While the seraphim joined in heavenly chorus, “Holy, holy, holy is the LORD of hosts; the whole earth is full of His glory!” (Isa. 6:3), the prophet of God cowered in painful recognition of his sin and peril in the presence of the holy God. “Woe is me, for I am undone” (Isa. 6:5). Yet while Isaiah was powerless to do anything to save himself, God acted on his behalf so as to declare, “Your iniquity is taken away, and your sin purged” (Isa. 6:7).
Our sin is never more glaring, never more grievous, never more hideous than it is in the light of the glory of God. The closer we draw to God, the more painfully aware we will become of our sin and shame. That realization is what lies behind John’s assertion: “If we say that we have no sin, we deceive ourselves, and the truth is not in us” (1 John 1:8).
Truth is in accord with the light. It testifies to the way of God and the glory of the gospel.