Let's be clear about a couple of things:
(1) Bernie Sanders is violating separation of church and state by attempting to enact a religious test.
(2) Bernie Sanders is an independent who has caucused with the Democratic Party and tried to get the Democratic Party nomination, but failed.
Sanders is not a good representative of the historic Democratic Party's positions. However, that may be changing since his failed bid to win nomination.
In terms of church and state issues, the Democratic Party has been a more consistent voice for religious liberty through institutional separation of church and state than Republicans, although there are a number of exceptions on both sides.
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To avoid some of the inevitable backlash for posting this, I think I need to state that I am not a Democrat, nor have I ever voting in any consistent way for Democrats (only if the Republican candidate was incompetent or corrupt - which tends to happen in a modest amount of state and local elections). I have been a moderate Republican, although I have such a strong distaste for the Trump Party (aka, "Republican Party") at the moment, I do not consider myself a Republican either.