Complex sentence: Geiger was not only a scholar and researcher commenting on important subjects and characters in Jewish history, he was also a rabbi responsible for much of the reform doctrine of the mid 19th century, contributing much of the character to the reform movement that remains today.
Split into simpler sentences: Geiger was not only a scholar and researcher commenting on important subjects and characters in Jewish history, he was also a rabbi responsible for much of the reform doctrine of the mid 19th century. He contributed much of the character to the reform movement that remains today.