Complex sentence: Within a few decades Gunpowder Treason Day, as it was known, became the predominant English state commemoration, but as it carried strong Protestant religious overtones it also became a focus for anti-Catholic sentiment.
Split into simpler sentences: Within a few decades Gunpowder Treason Day, as it was known, became the predominant English state commemoration. As it carried strong Protestant religious overtones, it also became a focus for anti-Catholic sentiment.