3 Are not Martyrs witnesses of Christ, and do they not bear witness to the truth? But if we think more carefully, when those Martyrs bear witness, He bears witness to Himself. For He dwells in the Martyrs, that they may bear witness to the truth. Hear one of the Martyrs, even the Apostle Paul; “Would ye receive a proof of Christ, who speaks in Me?” When John then bears witness, Christ, who dwells in John, bears witness to Himself. Let Peter bear witness, let Paul bear witness, let the rest of the Apostles bear witness, let Stephen bear witness, it is He who dwells in them all that bears witness to Himself. For He without them is God, they without Him, what are they?
Source: Sermons on Selected Lessons of the New Testament (New Advent)