Freedom of religion is the individual's right or freedom to hold whatever religious beliefs he or she wishes, or none at all. This freedom extends mere freedom of thought by adding the freedom of worship and the freedom of religious congregation, and is regarded as one of the basic human rights.
During history some countries accepted some form of freedom of religion (e.g. Poland or certain Muslim countries) but very often that freedom was abandoned. In most parts of Western society, freedom of religion was suppressed since the introduction of Christianity until the Enlightenment in the 18th century.
The Separation of Church and State is a related, but different concept.