Everything is pure to people who are pure. But if people are defiled or faithless, nothing is pure for them; even their mind and conscience are defiled.
They declare that they know God, but they deny him by what they do. They are detestable and disobedient, and useless for any good work.
First off, this verse (15) has been misused, abused, taken out of context to justify…whatever. Let’s be clear that Paul is referring specifically to Jewish food laws and ritual purity. Some people were saying that you had to keep Jewish food laws in order to become a Christian.
Well, all foods had been declared clean to eat, sooooo sorry ’bout your old eating requirements.
At the heart of this passage is what Jesus taught—that purity is a matter of inner disposition, not external performance, adherence to ritual, or avoidance of the “unclean”.
The reason everything was impure for those who Paul calls “defiled or faithless”, is because their minds and consciences were corrupted—the place out of which flows who you really are and how you see the world.
The word translated “mind” is the Greek word nous, for which we have no great English equivalent. You might say it is our spiritual receptive capacity, our deep understanding, receptive capacity of the intellect, or the comprehension. It’s like a little radio inside of you which can actually be tuned in to the voice of God (not necessarily audible). The nous can be trained to register or perceive God speaking and moving.
If that is off, everything is off, so that you see the good that God has made as impure or corrupt. Impurity is in their souls, not in God’s created world. Since their nous is defiled, everything appears impure to them, and, therefore needs legalistic regulation.
How ironic that “by avoiding what they consider ritually detestable, the false teachers become detestable themselves….with all their attention to minutiae of the Mosaic law, they are still found to be ‘disobedient’ because they mistakenly reject true Christianity (which centers on salvation by grace through faith) in favor of their own brand of religion.” (Andreas Kostenberger)
Professing to know God, they were not even fit for Kingdom work, since they did not first surrender their internal lives to Christ who announced and inaugurated said Kingdom.