As the Old Testament progressed, we saw how the Ark of Noah was a type of the Church. And just as at the time of Noah, we see that though everyone is invited to enter the Church (the Ark), not everyone is able to enter it. Noah and his family were able to enter the Ark, but not the rest of the populace outside. The gesture of God closing the door of the ark Himself, is declarative of His prerogative to choose whom He is to invite and enable to enter into the ark.
The Church was like an infant at the time of Adam and Eve, an adolescent at the time of Noah, and a young maiden at Pentecost. The Church is a live being. It grows. God is preparing her for her wedding with His Son at the height of her beauty and comeliness. She is destined not to grow old. And how she looks like at this wedding is described in the Apocalypse.
Let us take a look at the Liturgy of Pentecost. Liturgy tells us of the spiritual significance of each feast or part of the Gospel. It answers, too, two questions: how does the Church look like and how do we enter the Church. The answer to both questions is the plan of salvation.
On Pentecost day, how did the Church look like and, like the apostles, how do we enter the Church?
The Church is like a live human body with a soul. Her body is made up of the Saints in Heaven, the holy souls in purgatory and the living saints on earth (those who are still alive and have reached faith that is enlivened by Charity.)
The Church has a created soul, that is Jesus Christ, in His humanity. In the Old Testament, the Church in her infancy, only had a body, the patriarchs. It acquired its created soul with the coming of Christ. So the Church is now alive because she has a body and a soul.
Then at Pentecost, the Church received an uncreated soul, the Holy Spirit, which enabled its members to love God and neighbor in a supernatural way. "Love one another as I have loved you..." was impossible to the apostles until they received the Holy Spirit; the first instance of which is when Christ breathed on them before His Ascension and later on at Pentecost.
Seeing, therefore, the three parts of the Church, a body, a created soul and an uncreated soul, how do we become Catholic? By integrating ourselves into the Body, in which case the created soul of the Church becomes our soul and the uncreated soul of the Church becomes also our uncreated soul. How then do we integrate ourselves to the body?
First, by drawing near to Christ, as a broken branch is drawn near to the main Vine (as in the case of the parable of the Vine and branches.) And as this broken branch is drawn and attached to the main vine, so , too, by obeying the commands of Jesus Christ, are attached to the body, the Church.
Then, now that the branch is well attached to the Vine, the sap from the main Vine, which is the Holy Spirit, flows into the branch.
Now we see the the action of the Blessed Trinity : God the Father draws us, the broken branch, towards the Vine. "Unless the Father draws him ..." Jesus Christ attaches us to the main Vine. "I am the Vine ..." the Holy Spirit, the Spirit of Love, who flows from the Vine into the branches enables us to love God and neighbor.
Is it any wonder then that the feast of the Blessed Trinity follows immediately after Pentecost? What, then, is our role? Three things we must do: the first is hinted at in the Feast of the Trinity, the second is expressed at the next Sunday feast of Corpus Christi and the third step is described in the following feast of the Sacred Heart.