To which principle of government is montesquieu's statement most directly related?

First of all, Montesquieu is known for his statement that there should be a separation of powers within the government. He was known for his contributions for the aforementioned doctrine, however, we have to take note that he was not the one who was the author of it. He was referring to the separation of powers of the church and state, in this case, it applies in a democratic government - out of everything else.