“I do not ask for these only, but also for those who will believe in me through their word, that they may all be one, just as you, Father, are in me, and I in you, that they also may be in us, so that the world may believe that you have sent me.” (John 17:20-21)
At this point in His farewell to His disciples Jesus presents His final words in a form of a prayer which is often called His High Priestly prayer since in it He acts as intercessor with the Father. He asks for courage and faith for the disciples that none of them would fall away from faith though they would be sorely tested primarily because of the His crucifixion but in the long term due to persecution. He prays also for all who would believe in Him because of the message of the disciples, which includes us. He prays that we might share the unity He has with the Father. This unity is one we share with all believers through Christ our Lord.
As we persevere in our faith in Jesus we do so because of our union with Christ. This is because as High Priest Jesus offered not just a prayer. Like the priests in the Temple he would offer a sacrifice that would make possible the unity He has prayed for. But His sacrifice was the ultimate in atonement, the one that the Old Covenant sacrifices prefigured. The sacrifice was to be His body. His blood, not the blood of bulls or goats, would be poured out and sprinkled on the mercy seat for the sins of all mankind. This sacrifice would be performed once for all time though every time we partake of communion we share in His eternal offering.