The middle/passive voice of metamorphoō (G3213). Emphasizes that the transformation is something done to or within the believer by the Spirit — not something accomplished by human effort alone.
The passive voice in Romans 12:2 ('be transformed') and 2 Corinthians 3:18 ('are being transformed') is crucial: believers do not transform themselves. They submit to the Spirit's work of renewal. Yet it is not purely passive either — Paul commands 'be transformed,' implying cooperation. This is the mystery of sanctification: God does the transforming, but believers must actively yield to His work through spiritual disciplines, obedience, and the renewing of the mind.