Before the Church as you know it today existed, there was Jesus, his words and his examples.  Remember how many times he was surprised at the faith and insights of the non-Jews of the time.  (e.g. the centurian who wanted his servant healed, the foreign woman who stated that even the dogs eat crumbs the children drop from the table.)  Remember that he spoke to Samaritans which the Jews despised.  When the Samaritan woman asked where one should worship, Jesus told her that true worshippers worship in spirit and truth. He didn't order her to become a Catholic.  Perhaps these people eventually became Christians.  Who knows?  

The point is that Jesus accepted people where they were and touched their hearts.  Ecumenism is the work of Jesus because he desired that all be one.  Ecumenism may appear like compromising but look at what Jesus compromised to come and embrace us.

Remember that the Church is here for the sake of Jesus not Jesus for the sake of the Church.