 On 26 February 2001 he took the option open to cardinal deacons to be elevated to the rank of cardinal priest after ten years as a cardinal deacon.In 1999, Cassidy was jointly responsible for the publication of the "Joint Declaration on the Doctrine of Justification", widely received as an ecumenical peace document.Reflecting on his involvement, Cassidy joked that signing it would be the one thing he could confidently cite on his behalf on judgement day.