[16] "A little while and you will no longer see me, and again a little while later and you will see me." [17] So some of his disciples said to one another, "What does this mean that he is saying to us, 'A little while and you will not see me, and again a little while and you will see me,' and 'Because I am going to the Father'?" [18] So they said, "What is this 'little while' (of which he speaks)? We do not know what he means." [19] Jesus knew that they wanted to ask him, so he said to them, "Are you discussing with one another what I said, 'A little while and you will not see me, and again a little while and you will see me'? [20] Amen, amen, I say to you, you will weep and mourn, while the world rejoices; you will grieve, but your grief will become joy.
(CCC 2614) When Jesus openly entrusts to his disciples the mystery of prayer to the Father, he reveals to them what their prayer and ours must be, once he has returned to the Father in his glorified humanity. What is new is to "ask in his name" (Jn 14:13). Faith in the Son introduces the disciples into the knowledge of the Father, because Jesus is "the way, and the truth, and the life" (Jn 14:6). Faith bears its fruit in love: it means keeping the word and the commandments of Jesus, it means abiding with him in the Father who, in him, so loves us that he abides with us. In this new covenant the certitude that our petitions will be heard is founded on the prayer of Jesus (Cf. Jn 14:13-14). (CCC 2615) Even more, what the Father gives us when our prayer is united with that of Jesus is "another Counselor, to be with you for ever, even the Spirit of truth" (Jn 14:16-17). This new dimension of prayer and of its circumstances is displayed throughout the farewell discourse (Cf. Jn 14:23-26; 15:7, 16; 16:13-15; 16:23-27). In the Holy Spirit, Christian prayer is a communion of love with the Father, not only through Christ but also in him: "Hitherto you have asked nothing in my name; ask, and you will receive, that your joy may be full"(Jn 16:24). (CCC 2761) The Lord's Prayer "is truly the summary of the whole gospel" (Tertullian, De orat. 1: PL 1, 1155). "Since the Lord... After handing over the practice of prayer, said elsewhere, 'Ask and you will receive,' and since everyone has petitions which are peculiar to his circumstances, the regular and appropriate prayer [the Lord's Prayer] is said first, as the foundation of further desires" (Tertullian, De orat. 10: PL 1, 1165; cf. Lk 11:9).
Reference: thelema-and-faith.blogspot.com