Ukko is not the oldest Finnic name for the thunderer. Beneath him lies an earlier storm-god whose name, Perkele, is a loan from the same Baltic source as Lithuanian Perkunas, and Christianity, finding it a live pagan power, turned perkele into one of the strongest Finnish words for the Devil, where it remains a common curse. Ukko's own cult is attested in the spring festival Ukon vakat, "Ukko's bushels," a rite at which ale was drunk and offerings made to secure rain and a good harvest; his weapon is variously an axe, a hammer or a sword, and the rainbow was called Ukon kaari, "Ukko's bow."
The layering, a Baltic-derived Perkele overtaken by the native Ukko, then both overtaken by the Christian God, is a compact history of how one people's sky kept changing owners while the thunder stayed the same. The demonisation was fixed in print as well as in speech: early Finnish Bible translations rendered the Devil as perkele, so that the pagan thunder-name did duty as the standard word for Satan for centuries, until the 1992 revision of the Finnish Bible replaced it with paholainen. The same root spread the same fate across the region: Lithuanian Perkunas, Latvian Perkons, Prussian Percunis and Slavic Perun are cognate thunder-gods, and several of these names underwent a similar slide from divine name toward a darker sense once their peoples were Christianised.