John 3:18 He who believes in Him is not condemned; but he who does not believe is condemned already, …(a)
The greatest sin is unbelief. Anyone who dies in unbelief has their fate sealed. Think I of how Israel was very religious, they counted on their own righteousness to save them. They lacked the true knowledge that would lead them to Christ for their justification.
I see a direct correlation between John 3 and Rom 10. Here is Paul’s analysis and cry to God for men like Nicodemus and his people, “1 Brethren, my heart’s desire and prayer to God for Israel is that they may be saved. 2 For I bear them witness that they have a zeal for God, but not according to knowledge. 3 For they being ignorant of God’s righteousness, and seeking to establish their own righteousness have not submitted to the righteousness of God. 4 For Christ is the end of the law for righteousness to everyone who believes” (Rom 10:1-4). Unbelief was their downfall; they had fallen from grace (Gal 5:4). In biblical terms, “Fallen from grace” simply means trusting in Christ plus something or trusting on other things apart from Christ for your salvation. Their only hope was to abandon what they were trusting in to save them and submit themselves by faith to the righteousness of Christ ALONE. The just shall live by faith (Rom 1:17) for we are the righteousness of God by grace through faith in Christ Jesus.