1. What is the plan of God for man?
(cf. Catechism 1-25)
God, infinitely perfect and blessed in himself, in a plan of sheer goodness freely created man to make him share in his own blessed life. In the fullness of time, God the Father sent his Son as the Redeemer and Savior of mankind, fallen into sin, thus calling all into his Church and, through the work of the Holy Spirit, making them adopted children and heirs of his eternal happiness.
The plan of God for man is "to make him share in his own blessed life." This idea of sharing in the divine life is called by the Orthodox, theosis, deification or divinization. (I understand the connotations here, but it is not what the Orthodox mean.) It is most famously stated by St Athanasius (4th cent.), "For the Son of God became man so that we might become God." A rather bold statement, I should think, and yet there it is. This is the mystery of the incarnation.
Theosis, though the term is not often mentioned in Catholicism, is salvation. This is what it is about. This is what salvation means, what its purpose is. And this is our end. Theosis is the plan of God for man.
But I tremble for a qualification, so I will add St Athanasius' own: "becoming by grace what God is by nature." We do not "become God" in the sense that we become divine in our very essence (apotheosis is, I believe, still a heresy). We partake of or share in his divine nature. We become holy as God is holy. The doctrine of the Trinity certainly puts limits on the idea.
Here is a series of quotes in the Catechism, on the incarnation, which also speaks on theosis:
460 The Word became flesh to make us "partakers of the divine nature" (2 Pt 1.4): "For this is why the Word became man, and the Son of God became the Son of man: so that man, by entering into communion with the Word and thus receiving divine sonship, might become a son of God" (St Irenaeus, Adv. haeres., 3, 19, 1). "For the Son of God became man so that we might become God" (St Athanasius, De inc., 45, 3). "The only-begotten Son of God, wanting to make us sharers in his divinity, assumed our nature, so that he, made man, might make men gods" (St Thomas Aquinas, Opusc. 57, 1-4).