Not quite sure what you're going to end up with. Probably something sour. Not really beer.
For beer, you typically use malted barely. Not raw unmalted grain. You crush it, steep it in hot water (~150F ish), remove the grain, then boil it.
The malting process makes the starches available to be converted into sugars by enzymes in the grain. This conversion happens best in the presence of water in the ~150ish F temperature range.
The boil does a few things: it evaporates off some water, concentrating the sugars; it kills wild yeast and bacteria that naturally exist on the grain;
and it precipitates out proteins leaving a less hazy final product.
The now-sanitized-by-heat "wort" that results is then cooled to temperature that beer yeasts like. Yeast is added and the fermenting vessel is kept covered to prevent wild yeast and bacteria from contaminating the beer.
It's possible to open ferment with wild yeast, but your results will be unpredictable and most likely not especially tasty.
Shouldn't kill you, though