Abstract
The paper studies the disciplina arcani - an ancient practice of the Church which entailed speaking of some articles of the faith, the sacraments, and prayer only to those who were already initiated in the mysteries of the Church - in the Commentary on Psalm 118 of St Ambrose of Milan. In the Commentary, Ambrose uses several images that describe the acquisition of the divine knowledge and the content of the secret entrusted to the faithful that has to be kept safe.
First of all, there is the example of the Apostle John reclined on the chest of his Master, then the royal secret that has to be concealed, the riches and the inheritance to be guarded, the beauty of the Bridegroom that is better preached in silence than by words, and of the Christ-Illuminator who comes to shine in the soul of the believer though the latter finds himself in the darkness of night. These texts of the Commentary allow us to conclude that, for Ambrose, the content of the secret was not only the Creed, the Sacraments, or the Lord's Prayer but Christ's entire work of justification in the life of the faithful.
In other words, the secret to be guarded is Christ as present in the heart of the Christian and the believer's interiority as well. The source of this doctrine is not only the Alexandrian tradition, but Ambrose's classical education and culture as well.
Ambrose uses the Roman concept of trust (fides) that is gained by maintaining an entrusted secret and re-works it into a Christian concept of faith (fides) that is maintained by keeping and observing Christ's words which the Church entrusts to the baptized.