Here is the text for the Mizo song widely recognized as the first original Mizo Christian composition, "Mizo Kristian Hla Hmasa Ber" (The First Mizo Christian Song). This song was composed by Khuma in 1908.
The very first Mizo Christian hymn is widely acknowledged as “Isua Krista chu ka Pathian a ni” (Jesus Christ is my God). It was a simple, direct confession of faith, translated/adapted from a Welsh or English gospel song. For the first few years, this hymn—and a handful of others—existed only in handwritten notebooks or as oral repetitions. mizo kristian hla hmasa ber fixed
You mention "fixed" – meaning the standardized version we sing today. Here is the text for the Mizo song