He started speaking in tongues, believed by Pentecostals to be evidence of having received the baptism of the Holy Spirit after a series of meetings in Lillestrand.

Pethrus became pastor of a small Baptist church in Bengtsfors, Dalsland, Sweden, in 1903, but did not seek formal theological training until 1904, when he enrolled at the Bethel Seminary (Betelseminariet) in Stockholm.

After graduating from seminary, Pethrus completed his compulsory military service at Axevalla Hed [sv], Västergötland, in the autumn of 1906.

It was then that the doctrine of the baptism in the Holy Spirit and speaking in tongues became clear to him; he claimed: "From that moment I was part of the Pentecostal Revival".

During the same phase of his life Pethrus wrote the words and music for his gospel song "Löftena kunna ej svika" ('The promises cannot fail').

[1] The Filadelfia Church started a Bible school in 1915 and a weekly magazine, Evangelii Härold [sv] 'The Gospel Herald', in 1916.

In the same year, the church sent its first missionaries, Samuel and Lina Nyström, to Brazil, helping the Assembleias de Deus.