When Cecilia's angel places the crowns of roses and lilies on Cecilia and Valerian's heads, he tells them, "With body clene and with unwemmed thoght / Kepeth ay wel thise corones" (225-226). Then he tells them that unlike normal flowers, these crowns will never decay or lose their sweet smell.
The symbolism of these crowns is probably multiple: they represent Cecilia and Valerian's purity, since they are instructed to preserve this purity when they receive the crowns. The crowns also might represent a life that never decays or changes – eternal life in heaven with God. This interpretation fits well with Tiburtius's relationship to the crowns, which he is not able to see until he agrees to the truths of Christianity. If the crowns represent eternal life, they represent the reward of Christianity – this eternal life is something that, like the crowns, is unavailable, even invisible to non-Christians.