Christianity is a minority religion in Yemen. The constitution mentions religious liberty. There are about four thousand Catholics in Yemen. They belong to the Apostolic Vicariate of Arabia. The World Christian Encyclopedia, Second edition, Volume 1, states the Ethiopian Orthodox Church and the Russian Orthodox Church as largest denominations in Yemen. In Aden there are three Catholic and one Anglican church. In San'a there are weekly Protestant services. Christian missions from several countries are active in Yemen. There are hospitals that belong to foreign Christian institutions.