I believe it was Pope Pius XI (?) who lauded individual men and women living in the world, who have made a private vow of celibacy in order to serve God more freely.
Pope Pius XII in his Encyclical Letter
Sacra virginitas (
Acta Apostolicae Sedis, vol. xlvi., pp. 161-191; 25 March 1954) taught that perfect chastity "also flourishes among many who are lay people in the full sense: men and women who are not constituted in a public state of perfection and yet by a private promise or vow completely abstain from marriage and sɛҳuąƖ pleasures, in order to serve their neighbor more freely and be united with God more easily and closely,"
etiam apud non paucos floret, qui ad laicorum ordinem omnino pertinent; viri enim et mulieres habentur, qui in statu publico perfectionis non constituuntur, et tamen a matrimonio delectationibusque carnis ex proposito vel voto privato se omnino abstinent, ut liberius proximis inserviant et ut facilius arctiusque cuм Deo animum coniungant suum [translation of the Latin text is by the Vatican Polyglot Press].
The resolution to practice chastity according to one's state is the duty of all Catholic. The private vow of chastity of which the Pope speaks ought not to be taken by those committed to the single life without the counsel and tutelage of a canonically fit and trained Father Confessor.
Rev. Father Dominic Unger in his book
The Mystery of Love for Single People discusses the single life in light of the Holy Father's Encyclical Letter. The book was originally published by the Franciscan Herald Press in 1958 and has been reprinted by TAN Publications.