In September 1945, delegates of two separate Oneness Pentecostal bodies, the Pentecostal Church, Incorporated and the Pentecostal Assemblies of Jesus Christ, met in St. Louis, Missouri, and voted to merge into a single organization, first named the United Pentecostal Church. The union brought together two groups that had each carried the same distinctive teaching since the "Jesus' Name" controversies that split early twentieth century Pentecostalism: that baptism should be administered in the name of Jesus Christ alone, following the pattern of Acts 2:38, rather than in the Trinitarian formula of Father, Son and Holy Spirit, and that God is a single, undivided person rather than three coequal persons.
At the time of the merger the new church counted 521 congregations. Its own account of its history reports that growth since has been sustained and substantial: the UPCI today lists more than 45,000 churches and preaching points, some 45,000 credentialed ministers, and close to 6.2 million constituents, with an organized presence in 203 nations and territories. That places it among the largest Pentecostal denominations of any kind and the largest body within the Oneness, or Apostolic, wing of the Pentecostal movement specifically, a wing that remains institutionally and doctrinally distinct from Trinitarian Pentecostal denominations such as the Assemblies of God, with which it shares a Pentecostal emphasis on the baptism of the Holy Spirit and speaking in tongues but not the doctrine of the Trinity itself.