By craftiness in deceitful schemes

Heresies are made to look good so that we will believe them more readily.

So says John Owen:

It hath been the constant practice of all persons, in all ages, who have made it their design to beget and propagate a belief of any doctrine contrary to the form of wholesome words, to begin with, and insist mainly upon, those parts of their beloved conception and offspring which seem to be most beautiful and taking, for the turning aside of poor, weak, unlearned, and unstable souls.

...knowing full well that their judgments and attention being once engaged, such is the frame of men's spirits under delusion, they will choose rather to swallow down all that follows than to discharge themselves of what they have already received.

"Of the death of Christ" in The Works of John Owen: volume 10, p. 431