Aufer a nobis, quaesumus, Dómine, iniquitátes nostras: ut ad Sancta sanctórum puris mereámur méntibus introíre. Per Christum, Dóminum nostrum. Amen. [Take away from us our iniquities, we beseech Thee, O Lord, that we may be worthy to enter with pure minds into the Holy of Holies.] Roman Missal, Ordo Missae, Prayers at the Foot of the Altar
The first rite to open up the ordo of the Roman Mass to the influx of personal devotional prayer was that used by the papal household in the high Middle Ages. The earliest witness to the new direction this liturgy was taking has to do with the Office. This was Abelard in the year 1140 to St. Bernard of Clairvaux. As to the eucharistic celebration, some elements found at the beginning of this liturgical text and its end point toward the private chapel of the medieval Lateran palace, located at the head of the scala sancta. This chapel, preserved as renovated in 1278 has - from time immemorial - been called the Sancta Sanctorum, the Holy of Holies. This name came to be associated with the prayers at the foot of the altar, notably with their conclusion when the priest begs for the forgiveness of sins "so that, with sould made clean, we may be counted to enter the 'Holy of Holies'." With this prayer, which recurs in all of the oldest liturgical documents of the Roman Mass, we probably - in the final analysis - have one going back to the early sixth century at the very least. However, not until this venerable text was understood to allude to the local surroundings of the papal chapel did it become a prayer said during the procession of the pope, who - to offer the Holy Sacrifice - had to take himself from his palace apartments [in the Lateran] into the papal chapel. The prayer following in today's Mass ordo begs for God's clemency with the words, "by the merits of your saints, whose relics are here." These words, likewise, did not originally have to do with the holy remains of a saint deposited in just any altar, but rather with the vast treasury of relics found in the Sancta Sanctorum Chapel. It will soon be twenty years since the Jesuit Father Hartmann Grisar (1845-1932) undertook an examination of this treasury for the first time, an effort that led to conslusions of the utmost importance for archaeology and art history. Finally, the prayers of thanksgiving [Rorate note: including, "Da nobis, quaesumus Domine, vitiorum nostrorum flammas exstinguere; qui beato Laurentio tribuisti tormentorum suorum incendia superare. Per Christum Dominum nostrum."] said by the priest after the Mass point toward this chapel as well: the collect for the feast of St. Lawrence was incorporated into these prayers by virtue of the fact that the chapel is consecrated to the most revered martyr of Rome.
Anton Baumstark [the Younger]
On the Historical Development of the Liturgy
Now, many of the liturgical "certainties" of the first half of the 20th century are in doubt today, but, even if not exact, it is an interesting connection between the prayers at the foot of the altar and the Scala Sancta, this most revered relic of Rome, now under restoration: