In a letter to Bätzing, the doctrinal congregation said that the proposal did not do justice to the Catholic understanding of the Church, the Eucharist, and Holy Orders.
The CDF cautioned against any steps towards intercommunion between Catholics and members of the EKD.
In his open letter, Koch denied that he was refusing to engage with the ÖAK's arguments. He also highlighted what he called a "serious discrepancy" between the ÖAK's claims and common practice in Protestant churches.
The Swiss cardinal offered the example of the Protestant Church in Hesse and Nassau (EKHN), one of the member churches of the EKD. He noted that the EKHN invites people who have not been baptized to take part in the Lord's Supper service.
Koch said that this practice contradicted the ÖAK's claim that there is a "basic understanding" concerning an analogous "recognition" to baptism also in the "respective liturgical form of the celebration of the Lord's Supper."
"If, on the one hand, baptism and the mutual recognition of baptism are considered to represent the foundation of ecumenism, and if, on the other hand, an ecumenical partner relativizes baptism in such a way that it is no longer even a prerequisite for participation in the Lord's Supper, the question must be allowed as to just who is questioning the foundation of ecumenism here," Koch wrote in his open letter.
The cardinal said he was surprised "that such discrepancies between claimed ecumenical consensuses and the factual reality in the Protestant churches are not noted by the members of the ÖAK or, if they are, are not voiced, at least only in an extremely minimal way."
Koch nevertheless expressed gratitude that the ecumenical study group had invested "a lot of energy and passion" in overcoming questions that divide Catholics and Protestants. But he said that such steps could only truly succeed by confronting "concrete reality."
He added that unresolved questions should be named openly and then addressed.