Without knowledge of Christian doctrine, we are carelessly opening the way for sin, disguised as truth, to lead us away from the sound teachings of the Scriptures. This is the fate of the lazy man who can only bear to read “Christianity lite” in as far as it promises him gain for anything other than his soul. Augustus Montague Toplady writes:
But as for you, teach what accords with sound doctrine. (Titus 2:1 ESV)
[Doctrines] … are not of that small significance which the injudicious and the heterodox affect to give out. For, though matters of doctrine are, by some, considered merely as the shell of religion, and experience only as the kernel; yet let it be remembered that there is no coming at the kernel but through the shell: and, while the kernel gives value to the shell, the shell is the guardian of the kernel. Destroy that, and you injure this.
The apostle [Paul], … stamps the evangelical doctrines with the seal of dignity, usefulness and importance: as is evident from the epithet he makes use of. He calls the system of gospel-truths sound doctrine: (…Toplady again quotes in Greek…) salutary, health-giving doctrine; not only right and sound in itself, but conducing to the spiritual strength and health of those that receive it: doctrine, that operates like some efficacious restorative on an exhausted constitution; that renders the sin-sick souls of men healthy, vigorous and thriving; that causes them, through the blessing of divine grace, to grow as the lily, and to cast forth the root as Lebanon, to revive as the corn, and to flourish as the vine, to diffuse their branches, and rival the olive tree (Hos. xiv. 6) both in beauty and fruitfulness.
On the other hand, unsound doctrine has the very opposite effects. It impoverishes our view of God; withers our hopes; makes our faith languid; blasts our spiritual enjoyments; and lays the axe to the very root of Christian obedience. We may say of it, as the Jewish students said, on another occasion, there is death in the pot. If you eat it you are poisoned. With the utmost attention, therefore, should we attend to the apostle’s caveat, and avoid every thing that is contrary to sound doctrine. (“A Caveat against Unsound Doctrines”)