The following is from the pen of that great 19th Century evangelical bishop J. C. Ryle. Read his words and ask yourself if what he is saying is true or false. And if it is true, is that the way that your mind is trained to think? Or do you act as if what he wrote is false?
"Now this 'subtility,' St. Paul tells us, is precisely what we have to fear in false doctrine. We are not to expect it to approach our minds in the garment of error, but in the form of truth...The wolf would seldom get into the fold if he did not enter it in sheep's clothing."
"Such is the simplicity and innocence of many Churchmen in this day, that they actually expect false doctrine to look false, and will not understand that the very essence of its mischieviousness, as a rule, is its resemblance to God's truth...He goes into the church, expecting in his simplicity to hear nothing but heresy from the beginning to the end. To his amazement he hears a clever, eloquent sermon, containing a vast amount of truth, and only a few homeopathic drops of error."
"What discerning eye can fail to see that many Churchmen expect unsound teachers to be open vendors of poison, and cannot realise that they often appear as 'angels of light,' and are far too wise to be saying all they think, and showing their whole hand and mind. But so it is. Never was it so needful to remember the words, 'The serpent beguiled Eve by his subtilty.'"
J. C. Ryle, Warnings to the Churches, p. 130-1