"And do not mix up the truth with the falsehood, nor hide the truth while you know (it)" The Holy Quran, 2.42
Don't be deceived by truth Every successful con artist will provide some truthful information. They don't lie 100% of the time. Actually, most of what they say is truthful. The lies are a small portion of what they say Some of them will deceive you by simply omitting some important information. If you have a hard time understanding that concept, imagine a person telling you that your car was just stolen. He could be telling you the truth, but by omitting the fact that his friend stole your car, he is deceiving you.
Furthermore, they often provide very valuable information that you appreciate in order to fool you into being grateful to them