The Celts were all over Europe and were a serious competitor to the Roman Empire. Ultimately the Empire dissolved and the Celts remained, so in a sense the Celts won.
With a current global population of about a BILLION people with Celtic roots that seems like a fair assumption. {ggg}.
Respectfully, this part-celt disagrees. I think the celts failed to leave very much impact, due partly to lack of a written language at the time, and lack of a centralized government.
Also, they didn't survive as the cultural basis for any single nation. For example, modern Ireland is little more celtic, than it is anglo-saxon or norman or viking. Of course, I do grant that the language(s) remains with a not insignificant number of people, mainly in Ireland, Wales and Brittany.
An uncle of mine, born in the mid-1800s in Minnesota, of Canadian-Irish-protestant (eg. Scots) origins, was named Adomnan. This is the name of St. Columba's scribe, another priest at Iona. Scottish and Irish protestants can be good celts, too.