Complex sentence: Alexander had been imprisoned for his faith in the time of Roman Emperor Alexander Severus and on being released came to Jerusalem, where he was compelled by the aged bishop to remain, and assist him in the government of that see.
Split into simpler sentences: Alexander had been imprisoned for his faith in the time of Roman Emperor Alexander Severus. After his release, he came to Jerusalem, where he was compelled by the aged bishop to remain and assist him in the government of that see.