It is irrelevant that most of the Founders were religious. They deliberately established a secular, Constitutional, Republic. The very person who wrote the words you cite in the Declaration also wrote of a "wall of separation" between church and state.
Go on and sally forth with your Christian crusade. Just don't enlist the government.
I am not on a religious crusade, any more than one who advocates murder and stealing are morally wrong, for starters. Considering that those two were mentioned in the Ten Commandments, do we abandon the morality implicit for the purpose of shedding a religious context?
No, we don't. Yet the Founders did not require a religious test for holding office, and did not impose a specific religion on the masses, instead sought a guarantee that the People would be free to worship (or not) as they chose.
That did not keep the laws from reflecting that set of moral absolutes, any more than the influences of religious philosophers were abandoned in forming that same Republic. When people mention social issues, I must note the first villain you call on is organized religion, specifically Christianity, yet it that selfsame Christian philosophy which is strongly reflected in the writings, thoughts, and indeed, the Republic. Abandon that and the vacuum will be filled with something, and other ideologies as fervently adhered to as Christianity are available, from the Secular Humanism of the Marxists, to the 'kindness' of the eugenicists, to any of a host of other belief systems you might not find as equitable or forgiving as the Christian ethos.
So what do you propose to replace that with? A system which values only profit? one which imposes the redistribution of the fruits of the labors of those who work hard to those who will not? One which says murder is okay as long as your age is not between two arbitrary numbers? because when you stop dealing in absolutes, you now make everything negotiable, everything becomes a matter of 'pragmatism' or 'expediency', and no one, repeat, no one is safe from the depredations of the mob.