The above verse from Corinthians explicitly teaches against the heresy of ecumenism and is an unshakable defense against it. We are told here to be not only of one body, which the ecumenists strive for by trying to bring all religions into communion with one another, but also to "be perfectly joined together in the same mind and in the same judgement." This means that we must think and believe the same way. Ecumenism attempts to join everyone together into one body, while their minds, their beliefs, and their practices remain dissimilar and contradictory. This is not unity! How can there be one body but different minds? This is impossible! Colossians 1:18 states, "And he is the head of the body, the church." In order to belong to this body of which Christ is the head, we must be of one body, believing the same things. For as a head cannot have multiple bodies, so we cannot differ in beliefs yet belong to the same head. Ecumenism tries to make one church, but yet it does not truly unite. It turns a blind eye to differences in fath, belief, doctrine, and practice, as if these things do not matter. On the contrary, these things are of the utmost importance! These things are the very basis and foundation of our lives; they are the Church. If we cast these things aside, what is left? All that is left is a shallow, hollow shell of what was formerly the fullness of the Church. If we cast these things aside, we are casting aside our own salvation.

http://www.orthodox.net/articles/against-ecumenism.html