From Fr Spirago (in “The Catechism Explained”):
For the just heaven begins already on earth.
“While we seek life eternal, we already enjoy it,” says St. Augustine. The just have the true peace (John xiv. 27); they have the peace of God which surpasses all understanding (Phil. iv. 7); hence they are joyful even when fasting (Matt. vi. 17), and in the midst of sufferings (Matt. v. 12). The just possess the Holy Ghost, hence they are, even while still on earth united with God (1 John iv. 16). Christ ever dwells in their hearts (Eph. iii. 17); they have within them the kingdom of God (Luke xvii. 21).
From Fr Marin (in “The Theology of Christian Perfection”):
Heaven begun on earth. The saints desire to die to go to heaven, but in reality their life in heaven has already begun on earth. What do the things of this world matter to them? The servants of God live on earth only in their bodies, but their soul and their yearning are fixed in heaven. It is simply another way of stating the phrase: “But our conversation is in heaven” (Phil. 3:20).
Catechism (at no. 163):
163. Faith makes us taste in advance the light of the beatific vision, the goal of our journey here below. Then we shall see God “face to face”, “as he is”.47 So faith is already the beginning of eternal life:
When we contemplate the blessings of faith even now, as if gazing at a reflection in a mirror, it is as if we already possessed the wonderful things which our faith assures us we shall one day enjoy.48
Catechism (at no. 1000):
1000. Yet our participation in the Eucharist already gives us a foretaste of Christ’s transfiguration of our bodies:
Just as bread that comes from the earth, after God’s blessing has been invoked upon it, is no longer ordinary bread, but Eucharist, formed of two things, the one earthly and the other heavenly: so too our bodies, which partake of the Eucharist, are no longer corruptible, but possess the hope of resurrection.556
Catechism (at no. 1002):
1002. Christ will raise us up “on the last day”; but it is also true that, in a certain way, we have already risen with Christ. For, by virtue of the Holy Spirit, Christian life is already now on earth a participation in the death and Resurrection of Christ:
And you were buried with him in Baptism, in which you were also raised with him through faith in the working of God, who raised him from the dead . . . . If then you have been raised with Christ, seek the things that are above, where Christ is, seated at the right hand of God.559
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."560 The Father has already "raised us up with him, and made us sit with him in the heavenly places in Christ Jesus."561 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."562
1004. In expectation of that day, the believer’s body and soul already participate in the dignity of belonging to Christ.