In Umbris Sancti Petri

The Dogmatic Constitution, Lumen Gentium, speaks of the Church only in relation to the Trinitarian God; Father, Son and Holy Spirit. By beginning the document discussing the Father, Son and Holy Spirit, the Second Vatican Council is clearly showing that the sole reason for the Church's existence is to bring all people in union with this same God.

The first point begins by discussing God the Father and Creator of the universe, and His work in Salvation History with the following words: "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 world took place, wonderfully prepared in the history of the people of Israel and in the ancient covenant...” (Cyprian Epistle (Letters), 64, 4; Hilary of Poitiers, On Matthew, 23, 6; Augustine, passim.)

Then, the Fathers of the Council began to speak of the role of God the Son: "The Father, sent the Son into the world to inaugurate the kingdom of heaven on earth and the Church is the kingdom of Christ already present in mystery. The Church has it's origins in the blood and water which flowed from the pierced side of Christ crucified. (Jn 19:34)" “And I, when I am lifted up from the earth, will draw all men to myself.” (Jn 12: 32)

Finally, the Fathers discuss God the Holy Spirit: "The Father sent the Holy Spirit, on the day of Pentecost, after Christ His son had completed his earthy mission, to 'sanctify the Church continually and so that believers would have through Christ access to the Father in one Spirit (see Eph 2:18).'"

After the exposition of the Church's origins in the Most High God, the Council discusses the many rich imagines that Sacred Scripture has given us with respect to the Church:
- The sheepfold, whose single necessary door is Christ. (Jn 10: 1-10)
- The flock, “of which God Himself foretold that he would be the shepherd (see Is 40:11; Ez 34: 11 ff.), and whose sheep, even though governed by human shepherds, are continuously led and nourished by Christ Himself, the Good Shepard and Prince of Shepherds (see Jn 10:11; 1 Pt 5:4), who laid down his life for the sheep (see Jn 10:11-15).
- And many more images including the field of God, God's Building, God's Household, etc...

Perhaps the most moving image of the Church is that of the moon. The Church was spoken of often by the early Church Fathers as the moon, because all her light is from the Son, the light of the world. How clearly can we see that the Church is and belongs to Jesus?

Next, the structure of the Church is discussed along with it's identity as the Church Jesus founded 2,000 years ago. If the Church is the body of Christ, then there can only possibly be one Church of Christ, as Christ cannot possibly have two bodies. It is important that we remember the Church was founded by Christ and we are not able to change the hierarchical structure which He conferred on her. “The Church, set up and organized in this world as a society, subsists in the Catholic Church, governed by the successor of Peter and the bishops in communion with him.”