Poland–Lithuania, formally known as the Kingdom of Poland and the Grand Duchy of Lithuania and also referred to as the Polish–Lithuanian Commonwealth or the First Polish Republic, was a federative real union of the Kingdom of Poland and the Grand Duchy of Lithuania between 1569 and 1795. It was one of the largest and most populous countries of 16th- to 17th-century Europe. At its largest territorial extent, in the early 17th century, the Commonwealth covered almost 1,000,000 km2 (400,000 sq mi) and as of 1618 sustained a multi-ethnic population of almost 12 million. Polish and Latin were the two co-official languages. Catholicism was the state religion, though religious freedom was established in 1573.