This is taken from the J C Ryle Quotes page: http://jcrylequotes.com/
“What more common than to hear it said of some false teacher in this day:
‘He is so good, so devoted, so kind, so zealous, so laborious, so humble, so self-denying, so charitable, so earnest, so fervent, so clever, so evidently sincere, there can be no danger and no harm in hearing him. Besides he preaches so much real Gospel: no one can preach a better sermon than he does sometimes! I never can and never will believe he is unsound.’
“Who does not hear continually such talk as this? What discerning eye can fail to see that many Churchmen expect unsound teachers to be open vendors of poison, and cannot realize that they often appear as ‘angels of light,’ and are far too wise to be always 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, “Apostolic Fears”, 131.