For from within, out of the heart of man, come evil thoughts, fornication, theft, murder, adultery, coveting, wickedness, deceit, licentiousness, envy, slander, pride, foolishness. All these evil things come from within, and they defile a man. - Mark 7:21-23 (NAS)
And while I get what you're trying to imply being a "fool for Christ" as in:
For the preaching of the cross is to them that perish foolishness; but unto us which are saved it is the power of God. - 1 Corinthians 1:18 (KJV)
That's a far cry from Christians being a "foolish" people.
fool-ish adj.. 1. Lacking or exhibiting a lack of good sense or judgment; silly. 2. Resulting from stupidity or misinformation; unwise. 3. Arousing laughter; absurd or ridiculous. 4. Immoderate or subborn; unreasonable. 5. Embarrassed, abashed. 6 Insignificant; trivial.
American Heritage College Dictionary, Third Edition.
It's a word game. But your use of the word made it sound like we're supposed to be stupid.