Stocks rebounded Wednesday after Tuesday's action marked the worst start to September in 13 years. But widely followed investor Dennis Gartman said beware of the bounce.

"I'm afraid that this is a very real bear market and I'm afraid that most people are not prepared at all for it," Gartman told the "Fast Money" traders Tuesday. "This is very dismaying,"

The editor and publisher of The Gartman Letter is changing his tune after telling CNBC on Monday he was confused with the state of the equity market but was still buying stocks.

According to Gartman, the bull market actually came to an end six months ago, and now we're in the early stages of a full-fledged bear market.

"Rallies are to be sold," said Gartman. "End of discussion."

The crux of his argument was the strong correlation he found between oil and the equity market.

"Two weeks ago I was very bullish [on crude]—I'm not anymore," said Gartman. "If crude oil continues to fall, that will argue that demand is declining, that will argue badly for the economy in general and I think that's one of the coextensive indicators of a bear market in the equities market."

WTI crude oil fell 7.7 percent in Tuesday evening's trade, a significant move on the back of Monday's 8.8 percent leap higher.

Since 1984, WTI has only experienced back-to-back intraday swings this large five times. Gartman said investors should stay far away from the commodity.

He said $50 marks crude's new ceiling—something he attributes to the modern efficiencies of fracking.

"When people say fracking I say manufacturing. We have turned finding crude oil into a manufacturing operation," said Gartman. "As more frackable energy sites are discovered, the more obsolete oil will become. We're not going to see $100 WTI again in my lifetime."