St. Pius X allowed for Russian Greek Catholics to keep the saints on their calendar (presumably included saints after the Council of Florence) when they converted

St. Pius X allowed for Russian Greek Catholics to keep the saints on their calendar (presumably included saints after the Council of Florence) when they converted and Ven. Pius XII [specifically] approved 21 "Orthodox" saints for veneration and St. Paul VI added 4 more and in total added 25 "Orthodox" saints to the 1969 calendar revision

Source for those wondering: 

 Fr. Alphonse Raes, SJ's "La première édition romaine de la liturgie de S. Jean Chrysostome en staroslave," Orientalia christiana periodica 7 (1941): 518-526.

This article states (p. 518) that, in a 12/21/1934 motu proprio, Pope Pius XI commissioned the Sacred Congregation for the Oriental Church to publish liturgical books for Russian Catholics (AAS 1935:66). On the same page, Fr. Raes says that the first Roman edition of the Russian Greek Catholic Divine Liturgy of St. John Chrysostom was published in published in "Rome, Typographie de Grottaferrata 1940. In-8º, 112 pages." That is the only relevant Acta Apostolicæ Sedis citation in the whole article.

The liturgical calendar for Russian Greek Catholics omitted Peter of Moscow (1308-1326) and Alexis of Moscow (1354-1378) because they were consciously dependent on the Ecumenical Patriarch of Constantinople when he was clearly in schism from Rome (art. cit., p. 521). It also omitted Jonah of Moscow (1448-1461) and Philip of Moscow (1566-1568) because they rejected the Ecumenical Council of Florence (p. 521).

Why did the Liturgy include Nicetas of Novgorod (†1108), Leontius of Rostov (†1077), Barlaam of Khutyn (†1192), and Sergius of Radonezh (†1392)? Fr. Raes theorizes that they did not express schismatic sentiments (i.e., exhibited no pronounced anti-Catholic prejudices) and in their state of eremitic wilderness life, were probably never posed the question "for or against Rome?" (p. 521).

Some post-1054 saints common to Orthodox and Catholic calendars were, in fact, in communion with the Apostolic See, such as many 11th- and 12th-century holy monks from Kiev, according to Yves Cardinal Congar, OP in his, "A propos des saints canonisés dans les Eglises orthodoxes," Revue des sciences religieuses, 22 (1948), p. 246.

The following excerpt from the Roman Martyrology would verify the fact that Rome performed very thorough investigations into these post-schism individuals before determining whether they were to be called saints of the universal church.

"In monasterio Sanctissimae Trinitatis in Mosquensi Russiae regione, sancti Sergii de Radonez, qui, primum in silvis asperis eremita, dein vitam coenobiticam coluit et hegumenus electus propagavit, vir mitis, consiliarius principum et consolator fidelium" (Martyrologium Romanum, page 536).

"In the monastery of the Most Holy Trinity in the region of Moscow of Russia, of the holy Sergius of Radonezh, who was first a hermit in savage woods,..."

Pope Pius XII permitted 21 saints for veneration and they are from among the following 25 post-schism saints who appear in the General Roman Calendar of 1969:

1. St. Sava of Serbia (January 14) [1174-1237]

2. St. Nicetas of Novgorod (January 31) [†1108]

3. St. John the Martyr of Vilnius (April 14) [†1342]

4. St. Anthony the Martyr of Vilnius (April 14) [†1342]

5. St. Eustace the Martyr of Vilnius (April 14) [†1342]

6. St. Stephen the Enlightener of Perm (April 26) [1340-1396]

7. St. Stephen Pechersky (April 27) [†1094]

8. St. Cyril of Turov (April 28) [1130-1182]

9. St. Ignatius of Rostov (April 28) [†1288]

10. St. Isaiah the Wonderworker of Rostov (May 15) [†1090]

11. St. Euphrosyne of Polotsk (May 23) [†1173]

12. St. Leontius of Rostov (May 23) [†1077]

13. St. Nicetas the Wonderworker of Pereaslavl (May 24) [†1186]

14. St. German of Valaam (June 28) [†1353?]

15. St. Sergius of Valaam (June 28) [†1353?]

16. St. Anthony of the Kiev Caves (July 10) [983-1073]

17. St. Theodosius of the Kiev Caves (July 10) [†1074]

18. St. Theodore the Black of Yaroslavl (September 19) [†1299]

19. St. David of Yaroslavl (September 19) [†1299]

20. St. Constantine of Yaroslavl (September 19) [†1299]

21. St. Michael the Martyr, Wonderworker of Chernigov (September 21) [†1246]

22. St. Theodore the Martyr, Wonderworker of Chernigov (September 21) [†1246]

23. St. Sergius the Wonderworker of Radonezh (September 25) [1314-1392]

24. St. Abraham the Wonderworker of Rostov (October 29) [†1073]

25. St. Barlaam of Khutyn (November 6) [†1193]

Credit: Kyrylo Miazga