You may not know that, elementally, coal and diamonds are the same – made from carbon. As unlikely a pairing as you might imagine yet both known for their light – a light that shines in different ways.
Their expression of light is different but it is light – light that is neither immediately apparent nor easy to get to. The diamond needs to studied, mathematically evaluated to find the exact cut and facets; the coal is sorted, crushed and burnt; through the cracking and cutting the light gets out.
Such light is within each of us; as it was within Peter and Paul. They may be giants of the church – go to St Peters in Rome and see them standing 40 or 50 feet tall – but they are just like us; with all the faults and failings that humanity has to offer; which were offered into the fire of discipleship. They were never perfect – which gives us all hope. In fact it was their failings that saved them – that allowed Love to enter where reason and strength would not.