In 1920, the West ruled huge amounts of the world.
The West hasn't reached its universal state as yet, although its close to it, but it certainly has evolved out of its warring state phase, which it was in for a couple of centuries.
The British were white, English, and Protestant, just as we were. They had to have some other basis on which to justify independence, and happily they were able to formulate the inalienable truths set forth in the Declaration.
Partial truths or half-truths are often more insidious than total falsehoods.
It was this society and culture that among other things - including economic opportunities here and repression in Europe - attracted subsequent generations of immigrants to this country.
Total falsehoods can be easily exposed for what they are by citing exceptions to their claims. Hence, they are less likely to be accepted as the total truth.