So far as I am aware, Father Rahner was never judged by any competent court or tribunal in the Catholic Church as being a formal and pertinacious heretic. He may have been a material heretic as a private theologian, or may have held heretical ideas that he never published, but we have no way of knowing that.
If I am understanding correctly how theological study and analysis work, theologians are allowed to speculate on ideas and concepts that the average layman in the pew shouldn’t be musing about. In the end, if it comes to that, the Church then makes a decision yea, nay, or maybe.
As anyone who reads here regularly well knows, from time to time I dabble in a little speculative moral theology. (Full disclosure: I am toying, in the back of my mind, with going back to school — it would have to be distance learning — after my son graduates high school, and getting a degree in theology or counseling.) For instance, I have wondered if it would be morally licit for couples to use condoms,
solely for the purpose of not spreading or contracting a loathsome disease, with the contraceptive effect being totally unwilled and unwelcomed. You’ve got to admit that’s pretty progressive for an SSPX-sympathetic traditionalist
But I cannot speak authoritatively on any of this, and for now, it remains within my own mind — and if I were in those circumstances, I wouldn’t be entitled to invoke
in dubio libertas by saying “it’s a probable theological opinion”.
But to return to the point. Rahner has never been judged a heretic, and as far as I am aware, the Church has put no kind of
monitum on his writings (as she did with Teilhard). He does skate on the edge of Catholic orthodoxy. I couldn’t recommend his writings to the average Catholic who needs solid catechesis and orthodox doctrine more than he needs speculation and theological boundary-pushing.