In 1963 Karl Barth said: “The greatest obstacle to reunion between Protestants and Catholics is a little word which the Catholic Church adds after nearly every one of our Protestant affirmations. It is the little word ‘and.’ When we say Jesus, Catholics say ‘Jesus and Mary’. We seek to obey Christ, our only Lord: Catholics obey Christ and his Vicar on earth, the Pope. We believe the Christian is saved through the merits of Jesus Christ: Catholics add, ‘and our own merits’, that is, because of works. We believe the sole source of Revelation is Scripture: Catholics add ‘and Tradition’. We say knowledge of God is obtained by faith in His word expressed in Scripture: Catholics add and by reason.”
Barth concludes: “Practically the whole problem comes to discussing in what measure we can agree on the sense to be given to this little word ‘and’“.
(“Karl Barth: His Views on the New Oecumenical Climate,” The Tablet [March 2, 1963] 24.)