This is similar to what Jesus told them earlier this evening (13:19). It is not just a statement, but a prophecy. When the Holy Spirit does come to teach and remind them, and they receive the peace of Christ, they will believe. This last word means more than just believing the words He has said; they will more fully trust him when these things happen.

John 13:19  “I am telling you now before it happens, so that when it does happen you will believe that I am He.

Just exactly what is it that they will believe? They will believe the things that are about to happen and they will believe the spiritual significance of those things. They will believe the death of Christ and His burial in a tomb. They will believe His subsequent resurrection and ascension into heaven where He is now seated at the right hand of the throne of God the Father. They will believe that His death meant that we can die to sin and its penalty if we only accept what He did for us. They will believe that His resurrection meant that we can be resurrected to a new life in Christ and we can live in peace with God under the new covenant that Christ cut for us. And they will believe that His ascension into heaven means that He is interceding for us at every moment and that we, too, will go to be there with Him forever. That is what they are going to believe. Charles Wesley said it this way in the hymn “Christ the Lord is Risen Today”: “ours the Cross, the grave, the skies.” Those things are given to us to believe and to possess for our own. Thank You, Jesus!