"He who loves God cannot endeavor that God should love him in return." Baruch Spinoza (1632 - 1677), The Ethics
"Blessed are the weak who think that they are good because they have no claws."
Baruch Spinoza (1632 - 1677)

Spinoza was born in Amsterdam of Portuguese-Jewish parents. His character and circumstances were equally important to his philosophy, but he was almost the direct opposite of Leibniz. He was the heretic. When he was twenty-four, he was expelled from the Jewish community of his birth on account of his "abominable heresies." Later, the Christian authorities got in on the action and called him the vilest thing hell ever vomited on earth. On top of which, they pointed out, he was Jew.

There was something very rebellious in Spinoza, a fiery rejection of authority. At the same time, like some other heretics and infidels, he had the character of a true believer. He sacrificed everything in the pursuit of his philosophical vision. He was the perfect revolutionary, just as Leibniz was in a sense the consummate conservative.

Leibniz was heavily invested in the idea of a transcendent God—one who stands outside and before the world and creates it.
Spinoza's idea was of an immanent God—that is, a God who is identical with the world or nature itself.

Leibniz himself was a Spinozist of sorts, but he couldn't bring himself to believe that Spinoza's God was divine. An immanent or Nature God, he thought, wasn't a God at all. All of which left Leibniz—and God—in something of a pickle. He spent the rest of his life rehearsing the argument with Spinoza in his head, trying to squash his own inner Spinozist.

Because in Leibniz's view Spinoza's God was the worst of all possible Gods. Spinoza's God, as Leibniz saw it, is just a giant machine, utterly indifferent to the wants and needs of we little people. It doesn't think, eat, smell, or want anything; it isn't good or bad; it just is. Spinoza thought nature was divine; Leibniz didn't; and that was the real difference between them.

There is an element of tragedy in the stories of both thinkers. Leibniz was almost comically vain, greedy, and ambitious. He was the kind of man who is always angling for a better job, a fancier title, and more pay. At one point, he was holding down five jobs—and didn't bother to tell his various employers that he was moonlighting for the others. At the end of his life, his superiors finally got fed up with him, and his career took a nosedive. Not tragedy exactly, but more like farce. At the same time, there was something deeply honorable and sincere in everything Leibniz did. He really did want to make the world a better place. Unfortunately he believed that the way to do this was to re-create the unified religious and political order of the Middle Ages. Leibniz's life was a Don Quixote-style tragicomedy. He dedicated himself entirely to a project that, however virtuous in its conception, in the end amounted to nothing but vapors.

Spinoza lived and died in relatively tragic circumstances. He lived under constant threat of persecution, and he died young of lung disease that was arguably exacerbated by the lens-grinding work he was forced to take on. His "flaw" was his arrogance, his almost clinical level of self-sufficiency. Or maybe it was his complete inability to understand how other people just couldn't or wouldn't see things his way. He ended up a "tragic hero" of sorts—he sacrificed himself for the sake of helping establishing the modern, liberal, secular world order in which, by and large, we live today.

“We live in the best of all possible worlds.” – Gottfried Wilhelm Leibniz (1646 – 1716)
“We live in the worst of all possible worlds,and if it gets a little bit worse,it could not exist.”-Voltaire (1694 - 1778)
“If God did not exist, it would be necessary to invent him.”- Voltaire (1694 - 1778)
Voltaire was a "deist"--he was not an atheist, but he believed that God was an organizer, or a clockmaker, and that, after organizing the world and creating certain natural laws, he allowed it to run by itself.