I find the beginning of that sentence very important. It says "We hold these truths to be self-evident ..." No explanation needed; the truths speak for themselves; they simply are and have always been.
So just what truths are we talking about here?
First, that "all men are created equal" and that they are "endowed by their creator" with certain rights. You know life, liberty and the pursuit of happiness, that kind of thing. And men were granted and gifted with these rights by none other than the Creator, the same Creator that hung the stars in the sky. In other word, we come by them just because we exist.
Second, since these rights were given by the Creator, they could not be messed with.
They are unalienable, inalienable, and (for a short while in the earliest rough drafts) inherent. According to history.org, the unalienable rights that are mentioned in the Declaration of Independence could just as well have been inalienable (as in earlier drafts), but the word was changed when the document went to press. However, the words mean the same thing. They both refer to that which cannot be given away or taken away. The writers and signers of the document seemed to be fairly interested in making sure everyone knew that rights given by the creator were so important they could not be messed with.
As discussed before, the problem arises when we realize who the forefathers considered "all men." History tells us the term meant white males, most of whom were fairly well-to-do. For instance, how does slavery fit into "all men are created equal?" Or women not being able to vote; or "red skinned savages" being seen as disposable and in the way of our nation's destiny?
The document was signed and became the foundation for a new nation. Soon after the same man who wrote the main body of the Declaration became President of the United States and the westward push called "Manifest Destiny" began in earnest.
On the way to the Pacific we slaughtered most of the Native Americans we came in contact with; we pushed the Spaniards and Mexicans our of the Southwest; we dug for gold and silver and other metals and ores in the dirt and rock of the sacred lands of the tribes we encountered.
Finally, we settled the land from the Atlantic to the Pacific and there was no more room for expansion.
When expansion was slowed, the descendents of those who originally stated that "all men are created equal" fought a bloody war over the right to own other human beings. At the very least it was a war about the right for states to decide whether or not their citizens could own other human beings. Either way, it didn't say a lot for equality.
Needless to say, we've made some horrendous mistakes and even worse corporate decisions over the past 237 years. But we are getting closer to equality on many fronts. We are making strides.
That has always been one of our strongest and most positive traits. We keep pushing; we try to make things better; we try to live up to the ideals of the signers of the Declaration of Independence and the Constitution, even if they couldn't.
After all, this is America.