Jesus quoting the Shema shows he was a Jewish monotheist, which undercuts the Trinity. A man who says “the Lord is one” is not claiming to be God.

The Shema is cited to establish exactly that Jesus was a monotheist, because the Trinity is only worth arguing about if monotheism is non-negotiable. The interesting move happens elsewhere. In 1 Corinthians 8:6, Paul takes the Shema’s two key words, God and Lord, assigns God to the Father and Lord to Jesus, and still calls it one confession. Jesus affirming the Shema and the earliest church placing Jesus inside it are both data, and the Trinity is the attempt to hold them together without dropping either.