St John tells us in today's Gospel that when the disciples saw the Lord, they rejoiced. Seeing the Risen Lord gives joy, joy the world cannot know, joy that even the faithful cannot express, but only experience. That joy gives them to follow the Saviour and to go out on whatever mission He might send them.
St Gregory notes in his commentary on this passage: "The Lord sends His chosen Apostles, not to rejoice in the world, but to suffer in the world. Therefore, just as the Son is loved by the Father, and is yet sent to suffer; so likewise are the disciples loved by the Lord, and yet they are sent into the world to suffer. So it is rightly said, 'As my Father hath sent me, even so send I you': that is, When I send you forth into all the terrors of persecution, I love you with that love with which my Father loved me, when He caused me to come into the world to undergo my Passion".
We might do well to remember that in the midst of our paschal joys: The joy we experience in His resurrection is not the definitive joy of Heaven, but rather is destined to increase our love and confidence that, whatever might be the trials that lie ahead, the Risen Saviour holds us in the palm of His hand and will one day configure us to His glorified body, but only through the passion and the cross. Such is the deep meaning of the prayer: "Jesus, I trust you", popularised by St Faustina in the devotion to Divine Mercy. We trust the lead of God, and He leads us to Himself by means of suffering.
Believe and trust. And we might add paraphrasing St Augustine: Believe, trust, and then you will understand.