The Council of Trent was not an ecumenical council. It did not include bishops from the Eastern Church. It was small and limited to a select group of bishops which the Pope chose because he only wanted those bishops who agreed with him.

In 1530, when the Augsburg Confession was written, it was completely in line with the theology of the undivided church. There was nothing heretical about it. Calling a "council" of selectively chosen bishops in 1545 who were there only to oppose the Lutherans can't produce infallible teaching. Plus, there was no definition of what was and was not infallible teaching until Vatican I.

And--the Joint Declaration on Justification has thrown away the anathemas at Trent. Plus, Declaration on the Way has shown that much of what Trent said was based on misunderstandings and was time-bound.

After reading Declaration on the Way, what do you see as a mortal sin in someone moving to the Lutheran Church?