Every Catholic Church’s altar has a relic(a piece of bone) of some saint in it. If possible, the relic is from the saint that the Church was named after when it was built. If not possible, then the Vatican sends the relic of another saint. It isn’t necessarily a martyr.
The veneration of relics can be traced back to the earliest days of the Church. The Christians at Smyrna, wrote a letter, around AD156, describing the death of St. Polycarp, the disciple of St John the Evangelist.
After he had been burnt at the stake, his faithful disciples wished to carry off his remains, but the Jews urged the Roman officer to refuse his consent for fear that the Christians “would only abandon the Crucified One and begin to worship this man”.
Eventually, however, as the Smyrnaeans say, “we took up his bones, which are more valuable than precious stones and finer than refined gold, and laid them in a suitable place, where the Lord will permit us to gather ourselves together, as we are able, in gladness and joy, and to celebrate the birthday of his martyrdom.”
Our Church is a communion of Saints. The veneration of relics was recommended, and taken for granted, by writers like St. Augustine, St. Ambrose, St. Jerome, St. Gregory of Nyssa, St. Chrysostom, St. Gregory Nazianzen, and by all the other great doctors without exception.
We believe they are still alive in glory and stand before God as intercessory prayers for us all.