Complex sentence: It is believed that at least 100,000 people became Christians during the 1904--1905 revival, but despite this mass of people who turned to Christ it did not put a stop to the gradual decline of Christianity in Wales, only helding it back slightly.
Split into simpler sentences: It is believed that at least 100,000 people became Christians during the 1904-1905 revival. But despite this mass of people who turned to Christ the revival did not put a stop to the gradual decline of Christianity in Wales, it only held it back slightly.