The law of faith, being a covenant of free grace, God alone can appoint what shall be necessarily believed by everyone whom He will justify. What is the faith which He will accept and account for righteousness, depends wholly on his good pleasure. For it is of grace, and not of right, that this faith is accepted. And therefore He alone can set the measures of it: and what he has so appointed and declared is alone necessary. Nobody can add to these fundamental articles of faith; nor make any other necessary, but what God himself hath made, and declared to be so. And what these are which God requires of those who will enter into, and receive the benefits of the new covenant, has already been shown. An explicit belief of these is absolutely required of all those to whom the gospel of Jesus Christ is preached, and salvation through his name proposed.
Source:The Reasonableness of Christianity, as Delivered in the Scriptures (1695)