Jesus told the disciples a big call as well as big promises -- especially since Jesus said He would give them a place. However, Jesus made some announcements ahead of time so that the disciples can believe (14:29). Although the disciples may not understand everything going on, when they do understand, they can realize that Jesus was trustworthy.
Not only that, but Jesus would show the world that He loves the Father and that Jesus would do the commandment (14:31). In particular, Jesus is likely referring to his call to go on a cross to take on the penalty of sin on behalf of humanity. Jesus also told the disciples that "the ruler of this world is coming" and that "[the ruler] has nothing in [Jesus]" (14:30). The rulers capturing Jesus is how Jesus would later be on the cross.