I'm not a liberal, actually. Used to be a libertarian conservative, today call myself a democratic socialist. Was a member of a campus Bible study group back in college.
But .. it wasn't founded on "Christianity principles." It was mostly founded on "Enlightenment" principles, which called for religious tolerance and questioned religious orthodoxy. The Enlightenment was partly a reaction against the terrible, bloody, destructive religious wars between Catholics and Protestants from the 1500s through the early 1700s. Enlightenment philosophers, and the Founding Fathers who agreed with them, wanted to take religion out of government to avoid repeating the religious wars of the past. They didn't want Americans - who included Protestants and Catholics, Baptists and Quakers, Unitarians and at least a few Jews -- to kill each other over religion the way Europeans had a century earlier.
The liberals I know recognize this truth. They don't "hate the fact" that the US was founded on Christian principles, because they realize it's not a "fact."
-- democratic socialist for more honesty in both religious and political thinking