In the 1970s, I encountered the "Jesus Only" movement that originated from William Branham's 1960s teaching. The original members of the movement were called "Branhamites". They believed that there no trinity, but God was Jesus Christ who was also described as the Father and the Holy Spirit. This movement evolved into that now respectable movement called "Oneness Pentecostals".
The basic theological foundation of the movement is "Modalism" which describes the Father, Son, and Holy Spirit as different modes of the one God, so that when Jesus was on earth He appeared in that particular "mode", but before that, He operated in the "mode" of the Father, and presently He is indwelling us in the "mode" of the Holy Spirit. According to this theology, there is only one person who acts as a type of "shapeshifter" (in Science fantasy terms) like we saw on Star Trek Deep Space Nine, or Space 1999 which feature shapeshifting characters who could change their appearance depending on the circumstances. This is how the Modalist theology works.