Italian Archbishop Paolo Pezzi, 52, who has headed the Moscow-based Russia's Mother of God archdiocese since 2007, was reacting to growing public criticism of the Patriarch and Orthodox Church over their alleged excessive wealth and links with President Vladimir Putin.
In his open letter he added: "I express heartfelt sympathy at the many bitter ordeals and humiliations the Orthodox Church has recently had to bear from representatives of certain groups of society", the archbishop said.
The letter was published as Kirill pledged closer co-operation with Catholics.
Other minority religious groups have also declared support for the patriarch.