Just like anything else, one must be able to justify belief with reasoning and logic. Religion also experiences the debate between faith and reason, with a common misunderstanding that there is no need for rationality when discussing religion.
Contrary to this, Islam describes that rationality must be important for any true religion. It promotes critical thinking and explains that this is a God-given faculty, given specifically to humans so that they are able to understand and judge between right and wrong. To understand religion, it is important for one to be able to rationalise its teaching for that religion to have any effect on our lives.
No religion with any element of irrationality can be acceptable to the universal conscience of man. Further, Islam explains that reason alone is not sufficient to attain certainty in anything; reason must be aided by revelation. Reason can only get you to the hypothesis that there ought to be a Creator; revelation assists in arriving at the conclusion that there is a Creator. So, when reason is supported by an objective standard such as revelation, it is able to attain certainty through repeated empirical evidence.