Augustine of Hippo

(Tract. xxix. s. 6) Should any one however not understand this, let him hear the advice which immediately follows from our Lord: If any man will do His will, he shall know of the doctrine, whether it be of God, or whether I speak of Myself. What meaneth this, If any man will do His will? To do His will is to believe on Him, as He Himself says, This is the work of God, that ye believe on Him whom He hath sent. (c. 6:29) And who does not know, that to work the work of God, is to do His will? To know is to understand. Do not then seek to understand in order to believe, but believe in order to understand, for, Except ye believe, ye shall not understand. (Is. 7:9. Vulg.)