Is not believing in Jesus a sin?

I had never really thought of it that way before but in the last two days I have found Scripture that says exactly that!

John 15:18-25(NKJV)

“If the world hates you, you know that it hated Me before it hated you. 

If you were of the world, the world would love its own. Yet because you are not of the world, but I chose you out of the world, therefore the world hates you. 

Remember the word that I said to you, ‘A servant is not greater than his master.’ If they persecuted Me, they will also persecute you. If they kept My word, they will keep yours also. 

But all these things they will do to you for My name’s sake, because they do not know Him who sent Me. 

If I had not come and spoken to them, they would have no sin, but now they have no excuse for their sin. 

He who hates Me hates My Father also. 

If I had not done among them the works which no one else did, they would have no sin; but now they have seen and also hated both Me and My Father. 

But this happened that the word might be fulfilled which is written in their law, ‘They hated Me without a cause.’ 

John 16:5-11(NKJV)

“But now I go away to Him who sent Me, and none of you asks Me, ‘Where are You going?’ 

But because I have said these things to you, sorrow has filled your heart. 

Nevertheless I tell you the truth. It is to your advantage that I go away; for if I do not go away, the Helper will not come to you; but if I depart, I will send Him to you. 

And when He has come, He will convict the world of sin, and of righteousness, and of judgment: 

of sin, because they do not believe in Me; 

of righteousness, because I go to My Father and you see Me no more; 

of judgment, because the ruler of this world is judged.