From Dom Guéranger's The Liturgical Year.

THE holy Popes of the primitive ages of the Church abound during these last days of our Paschal season. To-day we have Felix I, a martyr of the persecution under Aurelian, in the third century. His Acts have been lost, with the exception of this one detail: that he proclaimed a dogma of the Incarnation, with admirable precision, in a letter addressed to the Church of Alexandria, a passage of which was read with much applause at the two (Ecumenical Councils of Ephesus and Chalcedon.

We also learn from a law he passed for those troubled times of the Church, that this holy Pontiff was zealous in procuring for the martyrs the honour that is due to them. He decreed that the Holy Sacrifice should be offered up on their tombs. The Church still keeps this law in mind by requiring that all altars, whether fixed or portable, must have, amongst the relics that are placed in them, a portion of some belonging to the martyrs. We shall have to speak of this custom in a future volume.

The Liturgy gives us this short notice regarding the holy Pontiff:

Felix Romanus, patre Constantio, Aureliano imperatore præfuit Ecclesiæ. Constituit ut Missa supra memorias et sepulchra martyrum celebraretur. Qui cum mense Decembri habuisset ordinationes duas, et creasset presbyteros novem, diaconos quinque, episcopos per diversa locaquinque, martyrio coronatus, via Aurelia sepelitur in Basilica quam a se ædificatam dedicarat. Vixit in pontificatu annos duos, menses quatuor, dies viginti novem.
Felix, a Roman by birth, and son of Constantius, governed the Church during the reign of the emperor Aurelian. He decreed that the Mass should be celebrated upon the shrines and tombs of the martyrs. He held two ordinations in the month of December, and made nine priests, five deacons, and five bishops for divers places. He was crowned with martyrdom, and was buried on the Aurelian Way, in a Basilica which he himself had built and dedicated. He reigned two years, four months, and twenty-nine days.

Thou, O holy Pontiff, didst imitate thy divine Master in his death, for thou gavest thy life for thy sheep. Like him, too, thou art to rise from thy tomb, and thy happy soul shall be reunited to its body, which suffered death in testimony of the truth thou proclaimedst at Rome. Jesus is the first-born of the dead;[1] thou didst follow him in his Passion, thou shalt follow him in his Resurrection. Thy body was laid in those venerable vaults, which the piety of early Christians honoured with the appellation of Cemeteries—a word which signifies a place wherein to sleep. Thou, O Felix, wilt awaken on that great day whereon the Pasch is to receive its last and perfect fulfilment: pray that we also may then share with thee in that happy resurrection. Obtain for us that we may be faithful to the graces received in this year's Easter; and prepare us for the visit of the Holy Ghost, who is soon to descend upon us, that he may give stability to the work that has been achieved in our souls by our merciful Saviour.

[1] Apoc. i 5.