While discussing the need for practical preaching - i.e., that both doctrine and duty should be preached - Charles Bridges, in his book, The Christian Ministry, argues that preaching doctrine doesn't deny one's practical obligations to that doctrine. He uses an argument from Bishop George Horne:
How we can be said to deny the existence of moral duties, because we preach faith, the root from whence they spring, I know not; unless he that plants a vine, does by that action deny the existence of grapes. The fruit receives its goodness from the tree, not the tree from the fruit, which does not make the tree good, but shows it to be so. So works receive all their goodness from faith, not faith from works; which do not themselves justify, but show a prior justification of the soul that produces them.Here's how I've outlined Hornes' thoughts:
We deny the necessity of works (fruit) because we preach faith.
If you plant a vine, do you deny the existence of grapes?
Fruit receives its goodness from the tree (not the tree from the fruit).
Fruit does not make the tree good, but rather shows that the tree is good.
Likewise, good works receive their goodness from faith (not faith by works)
Good works do not justify a man, but rather show that a man is already justified.
For by grace you have been saved through faith. And this is not your own doing; it is the gift of God, not a result of works, so that no one may boast. For we are his workmanship, created in Christ Jesus for good works, which God prepared beforehand, that we should walk in them.
(Ephesians 2:8-10 ESV, emphasis added)
But someone will say, “You have faith and I have works.” Show me your faith apart from your works, and I will show you my faith by my works.
(James 2:18 ESV, emphasis added)