CIC 556 On the threshold of the public life: the baptism; on the threshold of the Passover: the Transfiguration. Jesus’ baptism proclaimed “the mystery of the first regeneration”, namely, our Baptism; the Transfiguration “is the sacrament of the second regeneration”: our own Resurrection.1 From now on we share in the Lord’s Resurrection through the Spirit who acts in the sacraments of the Body of Christ. The Transfiguration gives us a foretaste of Christ’s glorious coming, when he “will change our lowly body to be like his glorious body.”2 But it also recalls that “it is through many persecutions that we must enter the kingdom of God”:3
Peter did not yet understand this when he wanted to remain with Christ on the mountain. It has been reserved for you, Peter, but for after death. For now, Jesus says: “Go down to toil on earth, to serve on earth, to be scorned and crucified on earth. Life goes down to be killed; Bread goes down to suffer hunger; the Way goes down to be exhausted on his journey; the Spring goes down to suffer thirst; and you refuse to suffer?”4

CIC 793 Christ unites us with his Passover: all his members must strive to resemble him, “until Christ be formed” in them.5 “For this reason we. .. are taken up into the mysteries of his life,. .. associated with his sufferings as the body with its head, suffering with him, that with him we may be glorified.”6

CIC 999 How? Christ is raised with his own body: “See my hands and my feet, that it is I myself”;7 but he did not return to an earthly life. So, in him, “all of them will rise again with their own bodies which they now bear,” but Christ “will change our lowly body to be like his glorious body,” into a “spiritual body”:8
But someone will ask, “How are the dead raised? With what kind of body do they come?” You foolish man! What you sow does not come to life unless it dies. And what you sow is not the body which is to be, but a bare kernel. .. What is sown is perishable, what is raised is imperishable. .. The dead will be raised imperishable. .. For this perishable nature must put on the imperishable, and this mortal nature must put on immortality.9

CIC 1003 United with Christ by Baptism, believers already truly participate in the heavenly life of the risen Christ, but this life remains “hidden with Christ in God.”10 The Father has already “raised us up with him, and made us sit with him in the heavenly places in Christ Jesus.”11 Nourished with his body in the Eucharist, we already belong to the Body of Christ. When we rise on the last day we “also will appear with him in glory.”12

CIC 2796 When the Church prays “our Father who art in heaven,” she is professing that we are the People of God, already seated “with him in the heavenly places in Christ Jesus” and “hidden with Christ in God;”13 yet at the same time, “here indeed we groan, and long to put on our heavenly dwelling.”14
[Christians] are in the flesh, but do not live according to the flesh. They spend their lives on earth, but are citizens of heaven.15

1 St. Thomas Aquinas, STh III, 45, 4, ad 2.
2 Phil 3:21.
3 Acts 14:22.
4 St. Augustine, Sermo 78, 6: PL 38, 492-493; cf. Lk 9:33.
5 Gal 4:19.
6 LG 7 # 4; cf. Phil 3:21; Rom 8:17.
7 Lk 24:39.
8 Lateran Council IV (1215): DS 801; Phil 3:21; 2 Cor 15:44.
9 1 Cor 15:35-37,42,52,53.
10 Col 3:3; cf. Phil 3:20.
11 Eph 2:6.
12 Col 3:4.
13 Eph 2:6; Col 3:3.
14 2 Cor 5:2; cf. Phil 3:20; Heb 13:14.
15 Ad Diognetum 5: PG 2, 1173.