[This] is how heresies have arisen. The heretics were never dishonest men; they were mistaken men. They should not be thought of as men who were deliberately setting out to go wrong and to teach something that is wrong; they have been some of the most sincere men that the church has ever known. What was the matter with them?
Their trouble was this: they evolved a theory and they were rather pleased with it: then they went back with this theory to the Bible, and they seemed to find it everywhere. If you read half a verse and emphasize over-much some other half verse elsewhere, your theory is soon proved.
Now obviously, this is something of which we have to be very wary. There is nothing so dangerous as to come to the Bible with a theory, with preconceived ideas, with some pet idea of our own, because the moment we do so, we shall be tempted to over-emphasize one aspect and under-emphasize another.
Source: D. Martyn Lloyd Jones, Studies In The Sermon In The Mount (Wm. B. Eerdmans Publishing Company; 1 v. ed edition, 1958)