Whoever does not believe God has made him a liar by not believing the testimony God has given about his Son. And this is the testimony: God gave us eternal life, and this life is in his Son. Whoever possesses the Son has life; whoever does not possess the Son of God does not have life.
1 John 5:10-12
We really do not have life if we do not believe Jesus is the Son of God. God gave us so much testimony to verify this, that when we do not believe him we do call him a liar.
That's like you telling someone something important and them not believing you. How does it feel to not be believed when we tell the truth. Think how God feels about now, when we don't believe Jesus is his son. How he has gone to great lengths through his prophets and witnesses to prove to us, that it is the truth. Why would he do that, he is God after all.
Yet we as human beings let so much darkness into our lives, that we forget how to truly trust in God. We doubt so much His love for us that we don't even try to possess the life that gives us life.
Believe me, you do not have life unless you have Jesus fully. I can say that I did not have life until I have fully let Jesus into my life to the point of doing His will and His Father's will. Life is so much sweeter and the hardships are truly not as bad as they once seemed. But life in Jesus is wonderful to have, you would not believe how magnificent it really is. God love you