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 2847 The Holy Spirit makes us discern between trials, which are necessary for the growth of the inner man,5 and temptation, which leads to sin and death.6 We must also discern between being tempted and consenting to temptation. Finally, discernment unmasks the lie of temptation, whose object appears to be good, a “delight to the eyes” and desirable,7 when in reality its fruit is death.
God does not want to impose the good, but wants free beings. .. There is a certain usefulness to temptation. No one but God knows what our soul has received from him, not even we ourselves. But temptation reveals it in order to teach us to know ourselves, and in this way we discover our evil inclinations and are obliged to give thanks for the goods that temptation has revealed to us.8

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 Cf. Lk. 8:13-15; Acts 14:22; Rom 5:3-5; 2 Tim 3:12.
6 Cf. Jas 1:14-15.
7 Cf. Gen 3:6.
8 Origen, De orat. 29 PG 11, 544CD.