So far the Hindus are concerned, and how they accept the Vedic knowledge as complete, here is an insignificant example. Just like the cow dung. The cow dung is the stool of an animal. According to smṛti, or Vedic wisdom, if one touches the stool of an animal he has to take his bath to purify himself. But in the Vedic scriptures the cow dung is stated as pure. Rather, impure place or impure things are purified by touch of the cow dung.
Now, if one argues how it is that in one place it is said that the stool of the animal is impure and another place it is said that the cow dung, which is also the stool of an animal, it is pure, so it is contradictory. But actually, it may appear to be contradictory, but because it is Vedic injunction, therefore for our practical purposes we accept it. And by that acceptance we are not committing mistake.
It has been found by modern chemist, modern science, one Dr. Lal Mohan Ghosa, he has very minutely analyzed the cow dung, and he has found that cow dung is a composition of all antiseptic properties. So similarly, he has also analyzed the water of the Ganges out of curiosity. So my idea is that Vedic knowledge is complete because it is above all doubts and all mistakes. So . . . and Bhagavad-gītā is the essence of all Vedic knowledge.