St. Felix ascended in 269 the throne of Peter to whom Jesus before His Ascension had committed His Church. He commanded Masses to be celebrated over the tombs of martyrs, and it is in remembrance of this prescription that the relics of martyrs are placed in a small cavity of the altar stone, called tomb. The altar, nowadays, has indeed often the shape of a tomb, this being a relic of the "Confession" or underground tomb which is found under the high altar in Roman basilicas, and is reached by stairs. The custom of uniting the remembrance of martyrs to the sacrifice of the Mass or of Calvary, shows that these martyrs, having entered into the bosom of Jesus (Gospel), have found there the strength to confess their faith before their enemies and the grace of being children of the Father (Epistle).
St. Felix bore witness to Christ in 274, under the persecution of Aurelian.
Protexisti me, Deus, a conventu malignantium, alleluja: a multitudine operantium iniquitatem, alleluja, alleluja. * Exaudi, Deus, orationem meam cum deprecor: a timore inimici eripe animam meam.
Thou hast protected me, O God, from the assembly of the malignant, alleluia: from the multitude of the workers of iniquity, alleluia, alleluia. * Hear, O God, my prayer, when I make supplication to Thee: free my soul from the fear of the enemy.
(Psalm 63:3,2 from the Introit of Mass)
Be mindful of our weakness, O almighty God, and since the burden of our deeds is grievous to us, grant that the glorious intercession of blessed Felix Thy martyr and bishop may protect us.
(Collect)
From the Catholic Encyclopaedia: http://www.newadvent.org/cathen/06029b.htm