Complex sentence: Moritz Daniel Oppenheim (1800--1880) was a German painter who is considered "the first Jewish painter" because his work was informed by his cultural and religious roots at a time when many of his German Jewish contemporaries chose to convert.
Split into simpler sentences: Moritz Daniel Oppenheim (1800--1880) was a German painter who is often regarded as "the first Jewish painter". His work was informed by his cultural and religious roots at a time when many of his German Jewish contemporaries chose to convert.