Considering that people do their oaths in so many different approved forms, its a bit of silly tradition that doesn't matter. If you don't want to make a religious oath, you're not forced to.
Please. Of course it's not the form of the oath which matters, nor, for this argument, does it matter whether or not anyone lies after taking it.
The oaths are a test case for revealing whether YOUR interpretation of the establishment clause would eliminate them, allow them, or not care -- and why. It's the "why" that might distinguish your interpretation from "I don't like it".
(footnote: the founder's views don't matter)