How the Pentecostal definition of tongues changed in the early 1900s.
The late Assemblies of God teacher, writer, and professor Gary B. McGee wrote a very factual account on the development and evolution of the doctrine of tongues in the Pentecostal movement.
It is a well-researched article with substantiated sources. One of the most definitive works found covering the subject from the late nineteenth century onwards.
He cites the most important leaders in the modern tongues movement, and how the original emphasis was on the supernatural acquisition of foreign languages. They perceived language learning as a long process and a barrier to a rapid missionary expansion throughout the world.
McGee outlined a serious flaw. The missionaries that went out with this expectation did not have this gifting upon arrival. They had to learn the language through the traditional method of study.
This forced a serious theological dilemma. Either the Pentecostal movement as a whole would have to admit they were wrong, or redefine the experience. They chose the latter.
McGee admits that somewhere between 1906 and 1907 the doctrine of tongues had changed from what was perceived as spontaneous language acquisition into worship and intercession in the Spirit:
Not surprisingly, though claims of bestowed languages had the potential of being empirically verified, such claims severely tested the credulity of outside observers. Corroborating testimony that Pentecostals preached at will in their newfound languages and were actually understood by their hearers proved difficult to find. By late 1906 and 1907 radical evangelicals began reviewing the Scriptures to obtain a better understanding. Most came to recognize that speaking in tongues constituted worship and intercession in the Spirit (Rom. 8:26; I Cor. 14:2), which in turn furnished the believer with spiritual power. Since on either reading–glossolalia for functioning effectively in a foreign language or for spiritual worship–the notion of receiving languages reflected zeal and empowerment for evangelism, most Pentecostals seemed to have accepted the transition in meaning.
It is surprising to find here that an Assemblies of God teacher admitted to this, though it comes across very softly.
Unfortunately, he failed to go into any details on what figures were responsible for this change, and how tongues as worship and intercession became an entrenched doctrine in such a short period.
The full document can be found here: Shortcut to Language Preparation? Radical Evangelicals, Missions, and the Gift of Tongues.