Ecumenism is heresy! That is quite clear. Moreover, it is the heresy of heresies.

Of all the errors that so-called "Ecumenism" comprises, the most fundamental and profound is its error concerning the very nature of the Church itself. This is an ecclesiological heresy. It is contrary to the Nicean-Constantinopolitan confession of faith, for it asserts that there is no "One, Holy, Catholic and Apostolic Church. 

(From the Monastery of Saint Shio of Mghvime: An Open Letter To The Patriach Of Georgia April 14/27, 1997)