The following quote from Fr Seraphim Rose is an only excerpt from the full article, but it summarizes what our Orthodox attitude must be towards non-Orthodox Christians.
“I will set forth briefly what I believe to be the Orthodox attitude towards non-Orthodox Christians…
1.) Orthodoxy is the Church founded by Christ for the salvation of mankind, and therefore we should guard with our life the purity of its teaching and our own faithfulness to it. In the Orthodox Church alone is grace given through the sacraments (most other churches don’t even claim to have sacraments in any serious sense). The Orthodox Church alone is the Body of Christ, and if salvation is difficult enough within the Orthodox Church, how much more difficult must it be outside the Church!
2.) However, it is not for us to define the state of those who are outside the Orthodox Church. If God wishes to grant salvation to some who are Christians in the best way they know, but without ever knowing the Orthodox Church—that is up to Him, not us. But when He does this, it is outside the normal way that He established for salvation—which is in the Church, as a part of the Body of Christ. I myself can accept the experience of Protestants being ‘born-again’ in Christ; I have met people who have changed their lives entirely through meeting Christ, and I cannot deny their experience just because they are not Orthodox. I call these people “subjective” or “beginning” Christians. But until they are united to the Orthodox Church they cannot have the fullness of Christianity, they cannot be objectively Christian as belonging to the Body of Christ and receiving the grace of the sacraments. I think this is why there are so many sects among them—they begin the Christian life with a genuine conversion to Christ, but they cannot continue the Christian life in the right way until they are united to the Orthodox Church, and they therefore substitute their own opinions and subjective experiences for the Church’s teaching and sacraments. “
– the Blessed Father Seraphim Rose
Read the full article here: http://www.pravmir.com/orthodoxy-on-other-christians/