The Council of Trent, referring in particular to the Holy Mass, motivated this habit by recalling that "human nature is such that it cannot be easily elevated to the meditation of divine things without external aids: on account of this the Church, as a loving Mother, has established certain rites.to render more evident the majesty of such a great sacrifice and to introduce the minds of the faithful, with these visible signs of religion and piety, to the contemplation of the sublime realities hidden in this sacrifice" (DS, 1746).

One of the oldest signs consists in turning to the east to pray. The east is symbol of Christ, the sun of justice. "Erik Peterson demonstrated the close connection between prayer toward the East and the cross, evident connection at the latest for the post Constantinian period.

We limit ourselves to conclude quoting Benedict XVI: "It is not necessary in prayer, and more than that, it is not even appropriate to look at one another reciprocally; much less so when receiving Communion. [...] In an exaggerated and misunderstood implementation of 'celebration toward the people,' in fact, the crosses at the center of the altars were removed as a general norm  -- even in the basilica of St. Peter in Rome -- so as to not obstruct the view between the celebrant and the people. However, the cross on the altar is not an impediment to sight, but rather a common point of reference.

"It is an 'iconostasi' that remains open, which does not impede being mutually in communion, but is a mediator and still signifies for everyone the image that concentrates and unifies our sight. I dare to propose the thesis that the cross on the altar is not an obstacle, but the preliminary condition for the celebration 'versus populum.' Also made clear with this would be the distinction between the liturgy of the Word and the Eucharistic prayer. Whereas the first is about proclamation and hence of an immediate reciprocal relationship, the second has to do with community adoration in which all of us continue to be under the invitation: 'Conversi ad Dominum' -- let us turn toward the Lord; let us convert to the Lord!" ("Teologia della Liturgia," p. 536).