>> in 337 ad, constantine the great, as he will come to be known, dies. >> with constantine and helena, what we're seeing is christianity moving out of the shadows into the mainstream but now into the dominant position. it's not just a religion. it's the religion. now the roman empire is going to be christian. the known world's going to be christian. >> but the question remains: did helena actually discover the cross jesus was crucified on? at the oxford university radiocarbon accelerator lab, archaeologist dr. georges kazan and professor tom higham are about to take a step closer to solving this mystery by carbon dating a fragment of the true