Error May Be Subtle over on The Watchman's Bagpipes is enlightening for any age:
...false teachings are often very subtle because they cross back and forth over truth. All cults and false teachers have some truth in their teachings, which makes discernment of the utmost importance. Often it is the true parts of their teachings that draws people unwittingly into the false, not being able to discern where the line was crossed
It is important for every Christian to be continually in the Word of God so that the line we follow is the true path, and so that we will be able to recognize the false teachings whenever they cross our line.