‘In all arts and sciences, logic, physic, mathematics, there are some praecognita, some rules and principles that must necessarily be known for the practice of those arts; so, in divinity, there must be the first principles laid down. The knowledge of the grounds and principles of religion is exceedingly useful.
(1.) Else we cannot serve God aright. We can never worship God acceptably, unless we worship him regularly; and how can we do that, if we are ignorant of the rules and elements of religion? We are to give God a ‘reasonable service.’ Rom 12:1f we understand not the grounds of religion, how can it be a reasonable service?
(2.) Knowledge of the grounds of religion much enriches the mind. It is a lamp to our feet; it directs us in the whole course of Christianity, as the eye directs the body. Knowledge of fundamentals is the golden key that opens the chief mysteries of religion; it gives us a whole system and body of divinity, exactly drawn in all its lineaments and lively colours; it helps us to understand many of those difficult things which occur in the reading of the word; it helps to untie many Scripture knots.
(3.) It furnishes us with armour of proof; weapons to fight against the adversaries of the truth.
(4.) It is the holy seed of which grace is formed. It is semen fidei, the seed of faith. Ps 9:10. It is radix amoris, the root of love. Eph 3:17. ‘Being rooted and grounded in love.’ The knowledge of principles conduces to the making of a complete Christian. (Thomas Watson, A Body of Divinity)