His Holiness Pope Benedict XVI during his General Audience, in the context of a week dedicated to Christian Unity spoke of the “four pillars” of Christian Unity.
After all, Benedict XVI is the Pope of Christian Unity.
From CNA with my emphases and comments.
Vatican City, Jan 19, 2011 / 12:55 pm (CNA/EWTN News).- Improving the unity of Christians today requires the same elements that united the first apostles in Jerusalem, Pope Benedict XVI said Jan. 19.
Pope Benedict met with pilgrims to Rome in the Vatican’s Paul VI Hall for his weekly general audience. In observation of the annual Week of Prayer for Christian Unity (Jan. 18-25), he based his message on “the gift of full communion.”
Christians take part in the week of prayer for unity “to bear witness to the profound ties that unite them and to invoke the gift of full communion,” said the Pope.
“They devoted themselves to the Apostles’ teaching and fellowship, to the breaking of bread and the prayers” is the theme of this year’s prayer week for the unity of Christians. Pope Benedict said that this passage from the Acts of the Apostles offers a vision of four characteristics that defined the first Christian community in Jerusalem “as a place of unity and love.”
[1 ] In the teaching of the apostles,  in fraternal communion,  in the breaking of bread and  in prayer are four “pillars” that continue to be the foundation of Christian life and build Church unity, he explained. [I must add: the Petrine ministry, unity with Peter.]
Every effort to increase unity must involve increased faithfulness to the teaching of the first Christians, the apostles, the Pope said. “Even today,” he explained, “the community of believers recognizes the norms of its own faith in that reference to the teaching of the Apostles.”
Fraternal communion was “the most tangible expression of unity between disciples and the Lord, especially for the outside world,” he pointed out. [I wonder if all of these can be brought into greater focus by looking at them with a view to the Petrine ministry. After all, the Church Jesus Christ founded has the Petrine ministry. We can't avoid it.]
Although it has not been without difficulty, the history of relations between Christians of all types is one of “fraternity, of co-operation and of human and spiritual sharing,” he said.
The Pope moved to the third characteristic, the “breaking of bread,” calling it the “pinnacle” of man’s union with God. As a way of unifying oneself with Christ’s sacrifice, he said, “it also represents the completeness of the unity of Christ’s disciples, full communion.” [He means, of course, the Eucharist... properly understood.]
Christians’ prayers take on a “penitential dimension” when one considers that at this moment it is impossible to share the Body of Christ with all Christians in the Eucharist, the Pope said.
He encouraged a “more generous commitment” to eventually bring Christians together in full communion, “breaking the Eucharistic bread and drinking from the same chalice.” [And how will we know when that is taking place?]
The final “pillar,” he said, is that of prayer. It means being open to the fraternity offered to Christians as the children of God, but also “it means being ready for forgiveness and reconciliation,” he explained. [To whom did the Lord give the keys of binding and loosing? Both of jurisdiction but also of forgiving sins? To the Peter, and to the Apostles with Peter.]
The Pope called for a “powerful witness” rooted in spirituality and supported by reason to be shared by all, as a message to those seeking clear points of reference in today’s world. [Peter is still a clear reference point. Christ wanted it that way.]
He underscored the importance of a constant increase in mutual love and an effort to overcome the difficulties that remain for full communion.
“We must collaborate as much as possible, working together on outstanding questions and, above all, being aware that we need the Lord’s help on this journey,” concluded the Pope. “He must still help us a lot because without Him, alone, without ‘abiding in Him’, we can do nothing.”
As it was pointed out in the combox, here is a strong passage which concerns the depositum fidei.
Still today, the community of believers recognizes, in the reference to the teaching of the Apostles, their own norm of faith: every effort made for the building of unity between Christians passes through the deepening of fidelity to the depositum fidei which the Apostles transmit to us. Firmness in the faith is the basis of our communion, it is the basis of Christian unity.
CNA missed this.