Correct construction

Who was the founder of Nichiren Buddhism in Japan? (in the 13th Century)

Who is the founder of the new religious sect recently started in Singapore? (recent past, founder still living)

Either could be correct, depending on if one is asking a question looking for a specific answer. Or if one were writing a piece on Nichiren Buddhism.

1. on a quiz - "Who is the founder of Nichiren Buddhism in Japan?"
2. a theme sentence in a paragraph - "Who was the founder of Nichiren Buddhism in Japan? The history books say ..."

The 'f' in founder is small case, unless it's a formal title or proper name.