In 1968 Pope Paul VI issued an encyclical letter forbidding artificial birth control.

He is best known as the author of one of the controversial treatises against the Greeks, called forth by the encyclical letters of Photius.

Apostolic letters are often scholarly theological documents, and they do not carry the same teaching authority as encyclical letters.

Since then, successive popes have added to and developed the Church's body of social teaching, principally through the medium of encyclical letters.

The encyclical letter issued by Pope John Paul II today is his seventh.

He also signed an encyclical letter, the 14th of his pontificate, intended to crack down on what he considers abuses of the eucharist.

The encyclical letter is a sign of solidarity with all clergy and faithful of the united Oriental Churches, who suffer for their faith in the East.

It also sets the theological framework for future encyclical letters, such as Mystici Corporis Christi (1943).

Some 50 years later, this created some controversy, as to whether the new Pope should have published the encyclical letter of his predecessor without changes or additions.

The encyclical letter certainly did not stop or shorten World War II.