This section lays out the big picture: the Church as part of God’s redemptive plan for humankind. You’ll notice that Lumen Gentium is heavily footnoted, particularly from Scripture, but no less from the early saints of the first five centuries.
The eternal Father, by a free and hidden plan of His own wisdom and goodness, created the whole world. His plan was to raise (human beings) to a participation of the divine life. Fallen in Adam, God the Father did not leave (people) to themselves, but ceaselessly offered helps to salvation, in view of Christ, the Redeemer “who is the image of the invisible God, the firstborn of every creature”.(Col. 1, 15) All the elect, before time began, the Father “foreknew and pre- destined to become conformed to the image of His Son, that he should be the firstborn among many (brothers and sisters)”.(Rom. 8, 29) He planned to assemble in the holy Church all those who would believe in Christ. Already from the beginning of the world the foreshadowing of the Church took place. It was prepared in a remarkable way throughout the history of the people of Israel and by means of the Old Covenant. (Cfr. S. Cyprianus, Epist. 64, 4: PL 3, 1017. CSEL (Hartcl), III B p. 720. S. Hilarius Pict., In Mt 23, 6: PL 9, 1047. S. Augustinus, passim. S. Cyrillus Alex., Glaph in Gen. 2, 10: PG 69, 110 A.) In the present era of time the Church was constituted and, by the outpouring of the Spirit, was made manifest. At the end of time it will gloriously achieve completion, when, as is read in the Fathers, all the just, from Adam and “from Abel, the just one, to the last of the elect,”(Cfr. S. Gregorius M., Hom in Evang. 19, 1: PL 76, 1154 B. S Augustinus, Serm. 341, 9, 11: PL 39, 1499 s. S. Io. Damascenus, Adv. Iconocl. 11: PG 96, 1357.) will be gathered together with the Father in the universal Church.
The Church prepared, constituted, and completed/gathered: it’s a nice picture of fullness. We remain today in a reality of incompleteness, awaiting a final gathering which will transcend the experience of time.