The disciples must have been very afraid at this point in their lives as they were just about to witness their Lord and Saviour tortured and killed. He was the power they knew and He was preparing them for a day that would affect them for the rest of their lives. He reassured them that He was going to be killed but He would not leave them powerless as they thought, but His Spirit and peace was promised to be given to them afterwards. He was not giving them a piece that would last for a short time, like the world gives, but one that would last till eternity. That peace would allow them to withstand anything thrown at them and kept them safe until He returned for them (only 3 days later). With that peace and with the trust they had in Him, He would leave them to spend 3 days and 3 night in the belly of the earth (Matthew 12:40).
When He returned as promised, He left them again with that promised peace and returned to His Father, where He would send a promised Holy Spirit that would lead them into all truth. We too have that promise, as He continue to do a great job, keeping us on the straight and narrow, assuring us that there is nothing to worry about. As Jesus has given us His Spirit and His peace, we need nothing else to cope, otherwise He would have given that too. He made sure we are taken care of until He returns, so be at peace, because all you really need to conquer the enemy is already embed in you. He will come back soon, do not be afraid, be at peace.