The problem is people built theology on that one verse.The same reason why the Greek word "Baptiso" was anglicised instead of translated "immersed". There are a lot of preconceived notions with Bible interpretations. Translators, when faced with a choice, are inclined to go with what is traditional. "Immersed" was never going to fly with the Roman Catholics and even the early reformers. "Was" likewise suited the prevailing theology. It is an acceptable translation.
I had an NIV study Bible many years ago. It had "became" as a possible translation in the margin notes. The theory is not new. Origen raised it in the third century.
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