‘He who believes in Him is not condemned, but he who does not believe is condemned already, because he has not believed in the name of the only begotten Son of God'.
He stresses that it is not God Who condemns men, rather they condemn themselves. When they see God's supreme Word, Jesus, revealing His glory and the glory of God, their very refusal to acknowledge Him condemns them. They are showing what they really are. For had their hearts been open and true they would immediately have believed in Him and received Him gladly. And their sin is made worse by the fact that that they are rejecting ‘the only Son of God'. This is then emphasised in another way.
‘He who believes in Him is not condemned'. What an incredible truth. For the one whose full trust is in Jesus Christ there can be no condemnation (Romans 5:1), for, because the Eternal Judge is also their Saviour, He makes intercession for them and points to His death on the cross on their behalf as proof that their sin has been atoned for. There is thus no one to lay a charge against one of God's chosen ones (Romans 8:33-34). But note that it is axiomatic that such a person turns from evil (John 3:19-21).
‘He who does not believe is condemned already.' This is the opposite side of the picture. What greater condemnation could there be than the rejection of God's offer of mercy. For their rejection of Him demonstrates the hardness of their hearts and their utter sinfulness.
‘Because he has not believed in the only begotten Son of God.' This in the end is why the condemnation is so great. It is not just anyone they are rejecting but the only true Son of God. It is almost incredible. The creature rejecting its Creator! Once again monogenes means someone of the same nature.