He who believes is not condemned
he who does not believe is condemned already
Your sin is not washed away if you do not have the faith of abraham
Jesus said this to a live audience before He had died and risen again to pay for our sins. The Gospels are narratives, not theological textbooks. The meaning is conveyed in accounts of real life events and things said are primarily time and circumstance specific. We cannot automatically assume that because something Jesus said during His ministry was true when He said it, that it would be true for all time and in all situations.
For instance Jesus said that the Father was drawing people to Jesus during Jesus ministry, but Jesus also said that if He was lifted up, He Himself would draw all men to Himself. So there was some change after the crucifixion and resurrection in the mechanism of God drawing people to Jesus.
Jhn 3:18 He that is believing (ὁ πιστεύων) into him (εἰς αὐτὸν) is not being condemned (οὐ κρίνεται, present passive indicative): but he that is not believing (ὁ δὲ μὴ πιστεύων) is being condemned already (ἤδη κέκριται, present passive indicative), because he has not believed (ὅτι μὴ πεπίστευκεν, present perfect active indicative) into the name (εἰς τὸ ὄνομα) of the only begotten Son (τοῦ μονογενοῦς υἱοῦ) of the
true God (τοῦ θεοῦ).
Applying this principle to John 3:17, Jesus was addressing a particular audience at a particular time, telling THEM that the person believing and who has been baptised has done enough
at this time in history is not under condemnation
at this time in history; but the person who is not believing
at this time is condemned already, because they have not believed into the name of Jesus of Nazareth.
This was the algorithm by which God was justifying or condemning people during Jesus' ministry. This algorithm was superceded by another, after Jesus was resurrected and glorified.