In several places Leibowitz writes that faith is not based on human knowledge but on accepting the yoke of the kingdom of Heaven.
For example (from “Paths to Faith in Judaism,” Ministry of Education and Culture, Department of Torah Culture, Jerusalem, 1981): “I do not see religious faith as a conclusion that a person reaches, but as a value decision that he makes; and like any value-content in a person’s consciousness, it does not arise from information supplied or given to him, but is an obligation that a person imposes upon himself” (exact quote).

I don’t know whether he means the claim “There is a God,” which is a factual claim, or whether he defines faith as commitment to God’s command. Factual claims are not a matter for decision but a conclusion. Nor is this a value question, as he writes. Commitment, of course, is a decision.