Change of heart and holiness of life, along with public and private prayer for the unity of Christians, should be regarded as the soul of the whole ecumenical movement... This love finds its most complete expression in common prayer... Fellowship in prayer leads people to look at the Church and Christianity in a new way.
Christ is present in unity because love is a union.
In Christianity the formal word for this is "ecumenism", which means "to inhabit one house."
One exciting aspect of ecumenism is that Jesus promises an experiential manifestation of his presence in worship when we agree in unity (Matt 18:19). Like little kids before the Father, we enjoy many answers to prayer as a result of being one heart in God, such as the charismatic manifestations of the Spirit (visions, tongues, prophecy, healing, etc.) which directly refresh us.
When history's greatest ecumenist began to undergo his passion, he requested his followers be united: "Father... may they be one as we are one: I in them and you in me... to let the world know that you sent me.” (John 17:21-23).
Thus, ecumenism begins with the honest recognition that today the true Body of Christ is torn.
Accordingly, the most influential leaders in Christianity echo Christ's plea. (E.g., "What unites us is much greater than what divides u"". Pope John XXIII.) See Ecumenical Agreement.
This does not imply that differences are unimportant ("false ecumenism"), but are secondary to the large, common core of Christianity, which makes us brothers and sisters in Christ—and neighbors in the New Jerusalem.
C.S. Lewis: "As a Christian, I am very much aware that our divisions grieve the Holy Spirit and hold back the work of Christ."
It also does not mean that individuals should compromise their denominational beliefs, but rather, like the U.S. and Britain in World War II, we form a strategic alliance against a mutual enemy.
Resolving theology must be left to the theologians and church leaders. And those leaders repeatedly encourage us to pray together and participate in environments such as charismatic prayer meetings where we can worship in common and work to build the Kingdom.
The key is to experience Christ in one another.