Adam Clarke's Commentary on the Bible

Verse 18 (John 3:18)

He that believeth - As stated before on John 3:16 .

Is not condemned - For past sin, that being forgiven on his believing in Christ.

But he that believeth not - When the Gospel is preached to him, and the way of salvation made plain.

Is condemned already - Continues under the condemnation which Divine justice has passed upon all sinners; and has this superadded, He hath not believed on the name of the only begotten Son of God, and therefore is guilty of the grossest insult to the Divine majesty, in neglecting, slighting, and despising the salvation which the infinite mercy of God had provided for him.

- Adam Clarke's Commentary on the Bible