Complex sentence: They also sang the alto parts in Handel's choruses, and it was as choral singers within the Anglican church tradition (as well as in the secular genre of the glee) that countertenors survived throughout the 18th and 19th centuries.
Split into simpler sentences: They also sang the alto parts in Handel's choruses. It was as choral singers within the Anglican church tradition (as well as in the secular genre of the glee) that countertenors survived as performers throughout the 18th and 19th centuries.