The enlightenment philosopher John Locke is commonly thought of as the first exponent of the doctrine of natural rights, However some trace it back further to the Roman Stoic Philosophers. The Stoics were concerned with right behavior and were respecters of religion but did not admit to the worship of any particular sect or deity.
They postulated that what set man apart from the animals was the divine spark which gave man all of his virtues. And that a man deprived of that spark by subjugation, as a slave, lost those divine virtues and thus became little more than an animal. It was reasoned then, by some that certain rights and freedoms were a necessary component of mankind.
Most people in this country, when they have heard of inalienable or natural rights think of them as God given. But it is not necessary to believe in God in order to accept the idea of preexisting rights.
Natural rights are preexisting, meaning that they are not granted by laws or the state, rather States and Laws are created in order to guarantee those rights. And Government is legitimate only if it protects those rights and rules with the consent of the governed.
Now the Left (liberal, progressive, socialist, call it what you will) Have always been in conflict with this doctrine. And not just because most of them are atheistic. They have always felt that there are no absolutes, therefore everything in the world must be viewed only in it's proper context. everything is relative.
This is of course a path to disaster. If, at any time the populace concedes that the Government has all of the rights and merely bestows them as it sees fit then it is a slippery slop down to tyranny.
It is not that the left don't believe in rights, but they are situational, and they are not always the same rights that the founders believed in. Free Speech is situational, religious freedom is really freedom from religion. Keep and bear arms? Guns are just icky. and by the way lets just add a bunch of junk and call it rights, like the right to health care or a free lunch.
As usual the roll of the lefty is to destroy. They would rather destroy our rights and replace it with something much much less.