Priest and professor of church music history at the Moscow Conservatory (b. 1818; d. 1889), born Dmitry Vasilyevich Razumovsky (Дмитрий Васильевич Разумовский).
Tchaikovsky greatly valued Razumovsky's advice concerning traditional Russian choral singing, and sought his advice when composing his liturgical works.
Correspondence with Tchaikovsky
2 letters from Tchaikovsky to Dmitry Razumovsky have survived, dating from 1875 and 1877, of which those highlighted in bold are available in English translations on this website:
2 letters from Razumovsky to the composer, dating from 1881, are preserved in the Klin House-Museum Archive.