Catholic theology
Catholic belief holds that the church exists both on earth and in heaven simultaneously, and thus the Virgin Mary and the saints are alive and part of the living church. Prayers and devotions to Mary and the saints are common practices in Catholic life. These devotions are not worship, since only God is worshiped. The church teaches that the saints "do not cease to intercede with the Father for us. ... So by their fraternal concern is our weakness greatly helped."
Apostolic succession
Roman Catholics recognise the validity of the apostolic successions of the bishops, and therefore the rest of the clergy, of the Eastern Orthodox, Oriental Orthodox, Church of the East, Old Catholic Church (Union of Utrecht only), and Polish National Catholic Church. The Orthodox generally recognise Roman Catholic clerical orders as being of apostolic lineage, but have a different concept of the apostolic succession as it exists outside the canonical borders of the Orthodox Church, extending the term only to bishops who have maintained communion, received ordination from a line of apostolic bishops, and preserved the catholic faith once delivered through the apostles and handed down as Holy Tradition. The lack of apostolic succession through bishops is the primary basis on which Protestant denominations (barring some like Anglicans and Old Catholics) are not called "Churches", in the proper sense, by the Orthodox and Roman Catholic Churches, the latter referring to them as "ecclesial communities" in the official documents of the Second Vatican Council.