AMERICANS, especially young ones, often forget that Thanksgiving is a United States holiday. In college, a friend once heard that two lads from New Zealand had no plans on this special Thursday. So she invited them and others over for turkey and fixings.Part way through dinner, someone suggested each say a few words about what Thanksgiving meant to them. "OK," chirped one of the New Zealanders, "But what's thanksgiving?" It hadn't occurred to the host that Thanksgiving isn't celebrated down under. Yet her motives were sound. Thanksgiving may be an American holiday, but its spirit translates to every language and culture. Thanksgiving is a day of love and sharing and a reminder of the simple, powerful idea of good: that good exists, and that, however humbly, it can be felt; that as part of the family of mankind, we can't do it all on our own but partake of a bounty outside ourselves. Americans, perhaps more than others, experience bounty. A friend from Romania reminded us again last week how wealthy the United States is compared with the struggling countries of Eastern Europe. There, enormous exertion is required to achieve even the smallest things Americans take for granted. This isn't cause for guilt. But it does make us ask about the depth of our gratitude. America, our Romanian friend feels, should be grateful for its "civil society," for a lack of vindictive anger among people - which is a problem in Romania today, he says. He wonders if Americans are really developing their enormous potential for good. Polls show that more than 80 percent of Americans believe in God - more than any other country. Yet thoughtful religious writers like Martin Marty say that belief is too often shallow. May it not be choked by mere acquisitiveness. Thanksgiving time, with its sweet smells of kitchen and smiles of loved ones, should be a time to deepen our roots.