Complex sentence: There was a consistently high membership at Heolyfelin, between 350 and 500, between 1860 and the First World War and the membership peaked in 1906, soon after the 1904-1905 Welsh Revival, during which 95 new members were baptised at Heolyfelin in December 1904.
Split into simpler sentences: There was a consistently high membership at Heolyfelin, between 350 and 500, between 1860 and the First World War and the membership peaked in 1906 in the immediate aftermath go the 1904-05 Revival. During the Revival, 95 new members were baptised at Heolyfelin in December 1904.