most wrongdoing is committed with some degree of knowing better. knowledge is usually the most relevant factor behind right and wrong actions (this is why there are different degrees of murder). the existence of a moral hierarchy implies that we are morally imperfect. it follows that although we all know better, none of us know best. if nothing can be ascertained, to dismiss something because of erroneous thinking by those who represent it proves that we can no sooner believe our own contrary conclusions.
knowing something that someone else doesn't is a one way street... you can know they don't know, but they cannot possibly know what they don't. humanity as a whole cannot know what it doesn't. you'd have to know everything to know that you know everything, and even the slightest bit less would leave you not knowing what you don't know. in the same way, the truth is absolute and something is either entirely true or it is simply untrue. truth and omniscience are one and the same.
the existence of partial knowledge and truth implies the existence of omniscience because either [absolute] truth exists or there is no such thing as truth. it could not be both.