To treat it as a religion and not a relationship is to fall into Pharisaism, going through rituals without either understanding or believing what it is that they signify. To treat Christianity as a relationship and not a religion is to fall into gnosticism, i.e. to disregard the sacraments and the Church that Christ founded, by which we receive grace and into which we, as material beings, are incorporated.
Protestants seem more liable to the gnostic error, as Jonathan discusses here. Sherry Waddell points out here that a fair number of Catholics tend toward to Pharisaism.
Imagine if we were united, and had the benefit of each other's gifts.
"Let unity, the greatest good of all goods, be your preoccupation." - St. Ignatius of Antioch (Letter to St. Polycarp)