To flesh out this aphorism, Cardinal-Priest Alfredo Ottaviani (1890-1979) was a Secretary of the Holy Office from 1959-1966 and prefect for the reorganized the Congregation for the Doctrine of the Faith.. During Vatican II, Ottaniani was a conservative voice against the Separation of Church and State and religious tolerance.
The other source Merton cites is about Pierre Tielhard de Chardin,(1881-1955) a Jesuit who was a French philosopher. Tielhard de Chardin's views on convergent evolution in The Phenomenon of Man (1955) were censored by the Catholic church for the views on original sin.
Merton's musing about progressed Catholics seems surprising today, as Merton is prized among the progressive faithful for his concerns for pacifism, social justice and outreach to other faiths. Yet Merton found progressed Catholics to be mean, inarticulate and secular.