Complex sentence: The Eldridge Street Synagogue is one of the oldest synagogues in the United States, and was the first one built in the United States by Eastern European Jews, who now make up the vast majority of American Jews.
Split into simpler sentences: The Eldridge Street Synagogue is one of the oldest synagogues in the United States. It was the first synagogue built in the United States by Eastern European Jews, who now make up the vast majority of American Jews.