I don’t remember transubstantiation being a major item in Luthor’s complaint against the church, although he did dispute it and the Lutheran church has a redefinition of what happens to the bread and wine that is called ‘sacramental union’ or something meaning that.
His main difficulties with the church seemed to be centered on indulgences and the works mentality of the RCC.
He disputed the claim that freedom from God’s punishment of sin could be purchased (indulgences).
Luther taught that salvation is not earned but is free gift paid for by the sacrifice of Christ. His theology challenged the papal authority by teaching that the Bible is the only source of divinely revealed knowledge. This is strongly debated to this day by the RCC in that they put tradition as another major source of divine knowledge equal to the Word.
Luther opposed sacerdotalism ? have to look that one up but it considers all baptized to be holy.
He really upset the church by getting the bible translated to German rather than just Latin!! This really grew the protestant church and lead to the further translations to English, etc.
He got married too ... something the RCC should do ... maybe that will be an outcome of this !! Halleluea!!!
Sad that he apparently became an anti-semite in his later years but that was probably more a creation of the times and had nothing to do with his theology ... I hope! ;-)