THE BOTTOM LINE
"Now we request you, brethren, with regard to the coming of our Lord Jesus Christ and our gathering together to Him, that you not be quickly shaken from your composure or be disturbed either by a spirit or a message or a letter as if from us, to the effect that the day of the Lord has come. (2 Thess. 2:1-2).
First, God has used him to show the world how foolish it is to date the future. It's one thing to "look for the blessed appearing of the glory of our great God and Savior" (Titus 2:13), it's another to circle a date on a calendar and hour on a clock. The former emphasizes our desire to be with Christ, the latter our desire to be seen as someone important with inside knowledge from God.
Second, God has used him to harden people in their arrogance. More than one atheist has patted himself on the back that he's more "enlightened." While he correctly assizes Camping as foolish, he mistakenly generalizes from that foolishness there is no return of Christ at all (2 Pet. 3:3ff.) And God uses Camping's foolishness to harden the atheist with that effect (2 Thess. 2:9-10). Laughing at the "fool on the hill" doesn’t equate to every Christian being a fool. We’re just not on the hill.
Third, God has used him to discipline well-meaning but gullible Christians to always test the prophetic. Rarely is naivete built on immaturity alone, but pride. People giving up jobs, spending their retirement, working/preaching long hours to get the message out is disheartening, but the rebuke should be evident -- humble yourself and listen to others outside your own circle. "The naive believes everything, but the sensible man considers his steps" (Prov. 14:15) -- a consideration that requires being teachable from others (Prov. 9:4, 16).
If Noah, a preacher of righteousness (2 Pet. 2:5), had concentrated his preaching on the 120 years God was to put up with mankind before the flood (Gen. 6:3) the unbelievers wouldn't have been caught off-guard when the flood came (Lk. 17:26-27); and two, he could have "dragged his feet" in building the ark, knowing he had 120 years. He did neither. He left the calendar -- and the math to God. So should we. There’s too much building yet to be done.
More links on Harold Camping and his false teachings can be found here.