“Jesus saves” is fine for a quick billboard type of broadcast.
But condemning the (erroneous) beliefs of those groups without the witness of the scriptures would be like Jesus shortening the Bible to Galatians 5:19-21
Details take time. Hence the need for preaching.
I’m not a fan of “message” t-shirts in part for the reason you state.
Still, kicking him out of over it is assymetrical with how other shirt-wearers are treated.