The first few years of Sigismund's reign (until 1598) saw Poland and Sweden united in a personal union that made the Baltic Sea an internal lake. However, a rebellion in Sweden started the chain of events that would involve the Commonwealth in more than a century of warfare with Sweden.
The Commonwealth at its maximum extent
The Catholic Church embarked on an ideological counter-offensive and Counter-Reformation claimed many converts from Protestant circles. The Union of Brest split the Eastern Christians of the Commonwealth. In order to further Catholicism, the Uniate Church (acknowledging papal supremacy but following Eastern ritual and Slavonic liturgy) was created at the Synod of Brest in 1596. The Uniates drew many followers away from the Orthodox Church in the Commonwealth's eastern territories.