Constantine, already burdened by many hardships, became ill. At one point during his extended illness, he experienced a vision of God and began to sing this verse: “My spirit rejoiced and my heart exulted because they told me we shall go into the house of the Lord.” Afterward he remained dressed in the vestments that were to be venerated later, and rejoiced for an entire day saying: “From now on, I am not the servant of the emperor or any man on earth, but of almighty God alone. Before, I was dead, now I am alive and I shall live for ever. Amen.” The following day, he assumed the monastic habit and took the religious name Cyril. He lived the life of a monk for fifty days.
From an old Slavonic life of Constantine