The very nature of the religion of the Old Testament, as a religion of revelation, implied a heavy leaning on the reading of the sacred books. The worship of the ancient Israelites naturally adopted the use of the Psalms from the very beginning. The use of such psalms was commonplace in every aspect of Jewish prayer, from the heights of the Temple, to the local synagogue and even within the lowliest household. As such, it was inevitable that the Church would also embrace the psalms which she would inherit from the Jews—songs which belonged to the person of the Messiah, who is her bridegroom. Before the Mass has ever begun, the fiftieth psalm (LXX) is recited during the rite of sprinkling with holy water. The psalm, whose antiphon begins with the ninth verse, “asperges me Domine hyssopo,” “you shall sprinkle me, O Lord, with a branch of hyssop” is selected to accompany this rite for the obvious imagery it invokes. As the sprinkling rite continues, the body of the psalm, verse 3, is recited, “Have mercy on me, God, according to your great mercy.” At the end of the rite, the Minor Doxology is sung, followed by the antiphon Asperges.
Historically, this has never been considered a part of the formal Mass, even from antiquity. It is possible that this rite developed in the West during the eighth or ninth centuries. Still, the recitation of Psalm 50 was always included in the private prayers at the foot of the altar before the beginning of Mass. From Easter to Pentecost, the antiphon verse 9 is replaced by the Vidi Aquam, “I saw water flowing from the Temple,” an allusion to Ez. 40. Likewise verse 3 is replaced by Psalm 117:1, “Give praise to the Lord for He is good.” This change is made in light of the events of the Passion as the Church moves from asking the Lord for His mercy, to rejoicing in having received it abundantly. The bond between the two sacraments of Baptism and Holy Communion is pronounced clearly in this rite. This is a visible reminder of our baptism and the necessity of purification from our sins. We may not receive the Lord unless He wash away the stains of our sins through baptism. The prayer is composed by a sinner, who feels the weight of his sins more than his sickness. The priest here relies on the Lord to be purified before he should dare to ascend the steps of the altar of sacrifice.
It is St. Augustine who properly explains the imagery that the psalm presents: “Hyssop we know to be a herb humble but healing: to the rock it is said to adhere with roots. Thence in a mystery the similitude of cleansing the heart has been taken. Do thou also take hold, with the root of your love, on your Rock: be humble in your humble God, in order that you may be exalted in your glorified God. You shall be sprinkled with hyssop, the humility of Christ shall cleanse you.” (Expositio Ps 50)