Had he lived longer, who's to say he wouldn't have decided to give his personal worldview a spiritual dimension? Or, alternatively, he could have let his cynical misanthropy get the better of him, and deliberately create a religion he didn't believe in himself. It's no coincidence I called it "pulling a Ron Hubbard". The existence of Scientology as a religion in OTL is evidence enough that when a SF writer starts either taking himself too seriously, or applying P.T. Barnum's motto that there's a sucker born every minute, he may well end up a latter-day prophet.
The problem is that HPL was the epitome of the "Scientific Materialist". If he couldn't taste, touch, hear, or see something, it isn't there. Anything beyond that is
purely speculation. You'd need to rework his entire character. Lovecraft allowed people to believe in his fiction because it might help him sell more copies, but he had a 'policy' that, if
anyone contacted him about it, he would set them straight.
However, that's not to say Lovecraft couldn't start a Cosmicist "faith". It wouldn't recognize Cthulhu, Yog-Sothoth, Azathoth or Nyarlathotep as hyper-dimensional alien entities, but it would recognize humanity's "grain of sand in the sahara" status in the universe.
...now, as far as people believing in his monstrosities, I sorta-count amongst their ranks. Do I think a Yog-Sothoth like beast exists? More than likely. But I don't think it cares..
at all, about us. Lovecraft "preached" cosmic
indifference. If anything, he was the world's most staunch nihilist.