Joh 16:30 Now are we sure that thou knowest all things, and needest not that any man should ask thee: by this we believe that thou camest forth from God.
Joh 16:31 Jesus answered them, Do ye now believe?
In the scene that this passage is from Jesus is giving His disciples instruction concerning what is about to happen. The discourse begins in Chapter 13 shortly after Judas is sent away to betray our Lord.
These 11 had walked with Him talked with Him, witnessed His miracles. They had heard His teachings, feed thousands with a few scraps, saw Him walk on water and stop a storm, even healed in His name yet they still did not believe; until now.
What changed was now it was personal.
Jesus opens this discourse with telling them they would soon come under persecution because of Him. He told them they would soon be without His physical presence and leadership.
Jesus told them He was going away to prepare a place for them. He told them that He and the Father where one, He told them that they would be one with Him. He told them that He would send the Holy Spirit to abide with them and in them, who would comfort them and teach them all things and bring all things to their remembrance.
He also, told them that if they believed in Him they would keep His commandments, that they would do the same works He did and even greater works. And, that He would do anything they asked to the Glory of the Father.
He Gave them one new commandment:
Joh 13:34 A new commandment I give unto you, That ye love one another; as I have loved you, that ye also love one another.
Joh 13:35 By this shall all men know that ye are my disciples, if ye have love one to another.
Some have argued that these instructions where for the eleven only but I think that John made it clear that Jesus was not speaking to just the eleven when he included the question from Judas who was not one the original twelve.(1)
About belief, Jesus said:
Joh 14:10 Believest thou not that I am in the Father, and the Father in me? the words that I speak unto you I speak not of myself: but the Father that dwelleth in me, he doeth the works.
Joh 14:11 Believe me that I am in the Father, and the Father in me: or else believe me for the very works'
What are those works today?
Hundreds of thousands upon hundreds of thousands of changed lives. Healing is still being done, though Lucifer has done a fine job of perverting the physical healing of the body through our lack of understanding and belief. But it does occur and you can find plenty of documented cases if you must in order to believe.
Not only that , there are the missions bringing not just the gospel to less fortunate but also food, clothing, shelter and medicines to the needy.
Christianity has been responsible for educating poor and establishing schools of higher learning.
Yes, Lucifer and his followers have been able to pervert these works and twist them to appear bad or evil and some of our well meaning but ill informed believers have given him the tools to do so, but that is no reason for disbelief . The good Christianity has brought our world far outweighs the bad of a few misinformed, ill behaved people. Even Jesus had a traitor in His group.
Let's take a look at Judas Iscariot for a second, He could not overcome his disbelief and became a tool of Satan's. Why couldn't he?
Some would argue that Judas was condemned from birth, that he was predestined to be the traitor. I don't think so. I think he had the choice to believe at any time, I believe that it was because of unrepented sin, something he felt he had to hold on to that blinded him from the truth. I also believed that Jesus knew that he would not repent but still gave him every opportunity to do so, which is why Jesus gave him the purse. Judas sin was pride, he lusted after power and riches and believed that Jesus would give that to him when Jesus became the King of Israel and the world.
Judas was not the only one of the twelve to have believed that the Christ was to be an earthly King. James and John believed that too (or at least their mother did) . but they changed (2).
Judas believed in the Christ but he misunderstood what the Christ's mission was, he thought the Christ was to be an earthly King, not a Heavenly or Spiritual King. He couldn't let go of his pride so he became a tool of Lucifer's and betrayed the Christ, Jesus our Lord.