Treatise on Grace

This is evident by what Christ says in John 3:6, where Christ, speaking of regeneration, says -- "That which is born of the flesh is flesh, and that which is born of the Spirit is spirit." Now, whatever Christ intends by the terms flesh and spirit in the words, yet this much is manifested and undeniable, that Christ here intends to shew Nicodemus the necessity of a new birth, or another birth than his natural birth, and that, from this argument, that a man that has been the subject only of the first birth, has nothing of that in his heart which he must have in order to enter in the kingdom. He has nothing at all of that which Christ calls spirit, whatever that be. All that a man [has] that has been the subject only of a natural birth don't go beyond that which Christ calls flesh, for however it may be refined and exalted, yet it cannot be raised above flesh. 'Tis plain, that by flesh and spirit, Christ here intends two things entirely different in nature, which cannot be one from the other. A man cannot have anything of a nature superior to flesh that is not born again, and therefore we must be "born again.