In fact, the “right to left” practice is likely the oldest.
The Sign of the Cross is a gesture by which Christians signify the blessing of their person in the name of the Father, the Son and the Holy Spirit. Other religions have similar practices, and it is difficult not to see in certain Jewish traditions the prefiguration of this particular Christian symbol, a physical, outward manifestation of a spiritual attitude. It is likely that this practice appeared very early in the history of Christianity, and it is striking that its evolution has followed that of the Church.
The tradition that has prevailed in the West and is customary among Latin Catholics is to make the Sign of the Cross by moving the fingers from top to bottom, then from left to right. Certain cultures join the five fingers, evoking the five wounds of Christ. But this custom is relatively recent and likely differs from the primitive practice, which is still prevalent in the Eastern Christian world. Indeed, in the beginning, Christians crossed themselves from top to bottom and then from right to left. The thumb, forefinger and middle finger were joined, evoking the consubstantial and indivisible Trinity, while the ring finger and the little finger folded against the palm of the hand evoke the two natures — human and divine — of Christ.
Like a blessing
The oriental and primitive tradition thus reproduces the gesture of the blessing given by the clergy as in a mirror: the blessing given by the priest or the bishop reproduces the gesture of Christ figured on Byzantine icons, where the thumb of the hand that blesses joins the ring finger, the index finger is upwards, the middle finger and the little finger slightly folded. Thus the hand of the priest forms the first letters of the words Jesus Christ in Greek — IC and XC — while recalling the association of the three persons of the Trinity and the two natures of Christ. The hand that blesses thus traces the Sign of the Cross toward the faithful from top to bottom and from left to right. This movement, which is always that of the blessings given by the clergy in the East as in the West alike, is probably that which has been used from the earliest times.
And this is what explains the Eastern practice of the Sign of the Cross: by reproducing as in a mirror the gesture of the blessing granted by an authority, it is a way of recognizing that one cannot validly bless oneself and that even in the absence of a member of the clergy, every blessing is received from a higher power, in other words from God.
The history of the Sign of the Cross is therefore also linked to that of the blessing. The current Western practice is to bless with an open hand, just as the faithful make their sign of the cross with their hand open.
Having become a marker of the specificities of each tradition, the preference of one direction or the other finds several symbolic explanations that are more or less convincing: drawing on oneself the cross from left to right, could signify the passage from darkness to light, from misery to glory, as Pope Innocent III speculated in his time, when this practice began to take place in the West in the 18th century.
But why has the blessing always been given from top to bottom and from left to right, dictating the original direction of the Sign of the Cross performed by the faithful? Perhaps just because it is the natural direction of reading and writing in Latin and Greek. In blessing, the clergy “write” the cross on those whom they bless. One thing is certain: the Sign of the Cross, whatever the direction in which it is traced, is a gesture of prayer and a petition to the Holy Trinity.