Why was James II's support of Catholicism and Catholic Leaders an issue?

Well, honey, let me break it down for you. James II's support of Catholicism and Catholic leaders was a problem because England was mostly Protestant at the time, and they weren't too keen on having a Catholic king cozying up to the Pope. It stirred up a whole lot of political and religious drama, leading to his eventual downfall and the glorious Revolution of 1688.