He that believeth on the Son hath everlasting life: and he that believeth not the Son shall not see life; but the wrath of God abideth on him. John 3:36 KJV
We have here in this verse two contrasting ideas. That of belief, and that of unbelief. It is very helpful to our understanding of the biblical concept of faith or belief, if we understand what is meant by unbelief.
The Greek word translated "believeth not" is apeitheo. The meaning of this word is to disbelieve (wilfully and perversely): not believe, disobedient, obey not, unbelieving. (Strong's definition)
Thayer defines it as, not to allow oneself to be persuaded. to refuse or withhold belief. to refuse belief and obedience. Not to comply with.
We can see from these definitions, that unbelief carries the idea of refusal to submit and obey. Therefore, to not believe in Christ is to refuse to obey Him.
The NASB puts it this way,
He who believes in the Son has eternal life; but he who does not obey the Son will not see life, but the wrath of God abides on him. John 3:36 NASB.
By this we can see that to believe in Jesus is much more than a mere head knowledge. It means far more than agreeing with the fact that Jesus exists, that He died and rose again. To believe in Jesus means that we commit ourselves to Him; that we surrender ourselves to His lordship over our lives, submitting to Him with a willingness to serve and obey Him.
It is a mistake to think that one only needs to believe in Jesus, but not obey Him. Yes, we believe He died and rose on our behalf, that He took our sins upon Himself, and is our only means of salvation. This is core to our faith. But if our faith does not submit to Him in obedience to His word, it is not faith at all.
For another article on unbelief and disobedience click here.