He did this by appearing to go through death of the body, and yet being not that body was able to make that body disappear, and then re-materialized another body image which looked and felt just as "real" as before, to SHOW that what He was is beyond and has mastery over the body and over death.
Can you imagine being there as Jesus "as a body", who was just crucified and supposedly killed, which you saw with your own eyes and believed, suddenly materializes out of nowhere as though nothing had happened? Imagine how much of an EDUCATION that would be, blowing your mind and totally undermining your belief system in which the body is thought to be real and all physical effects are thought to be irreversible. This is why miracles are teaching devices.
Nothing could happen to what He is, because He is Spirit, not body, and He was able to PROVE that this is true by dematerializing and rematerializing the body. Who and what He is MUST be "not a body" if the body is able to be switched on and off at will, and therefore anything the body experiences (such as death) cannot be real or have any power to do anything.
WE are also asked to DEMONSTRATE THE TRUTH through our seemingly miraculous ability to completely defy all laws of the physical world. What Jesus did was miraculous. Miracles are expressions which come from a higher state of awareness/mind/spirit, outside of spacetime, outside of the universe, manipulating what is "inside" the universe, to prove that the world is not real or true. Another word for this is SUPERNATURAL.
Jesus asks us to demonstrate, for example, that we are not sick in the body, and that our brother has not done anying to us to cause us harm. We can only do this supernaturally. We have to have the power of God working with us to defy the ego's laws of biology, physics, chemistry etc, to show that even if someone tries to harm us, we cannot be harmed. We demonstrate this USING the body as a teaching device, just as Jesus did.
Miracles in A Course in Miracles are supernatural ACTS, effects or results which are considered IMPOSSIBLE in the ego's thought system. Their "shifts into invisibility" are shifts away from the level of form (physical world) and towards the One Mind. They transcend the worldly laws and enable you to do LITERALLY ANYTHING. There is NOTHING your holiness cannot do! NOTHING!
Miracles are not shifts in perception or mere changes of mind. Miracles are possible AFTER mind has been restored to its natural state of miracle-mindedness. And the miracle-working ability remains a PERMANENT FEATURE of the Son of God, NOT a temporary measure of restoration with no power to do anything.
If you are to truly learn A Course in Miracles and recognize and prove what miracles are, you MUST become a supernatural miracle worker, OTHERWISE you will remain confined to the body and subject to death, determined to prove and demonstrate that the world has power over you and there is nothing you can do about it. That is nothing short of a state of victimhood and unconsciousness.
We need to becoming open to, and capable of, performing miracles to demonstrate and prove that God's truth is true, by defying the laws of the world, right here in the dream. If you are not willing to ACT on your belief that God's truth is true, then it is not true for you. Your miraculous acts will demonstrate that the truth is true, and destroy the ego's illusions that the body has power over the mind.