39 It is impossible to believe on our own. Faith is not simply an individual decision which takes place in the depths of the believer’s heart, nor a completely private relationship between the "I" of the believer and the divine "Thou", between an autonomous subject and God. By its very nature, faith is open to the "We" of the Church; it always takes place within her communion. We are reminded of this by the dialogical format of the creed used in the baptismal liturgy. Our belief is expressed in response to an invitation, to a word which must be heard and which is not my own; it exists as part of a dialogue and cannot be merely a profession originating in an individual. We can respond in the singular — "I believe" — only because we are part of a greater fellowship, only because we also say "We believe". This openness to the ecclesial "We" reflects the openness of God’s own love, which is not only a relationship between the Father and the Son, between an "I" and a "Thou", but is also, in the Spirit, a "We", a communion of persons. Here we see why those who believe are never alone, and why faith tends to spread, as it invites others to share in its joy. Those who receive faith discover that their horizons expand as new and enriching relationships come to life. Tertullian puts this well when he describes the catechumens who, "after the cleansing which gives new birth" are welcomed into the house of their mother and, as part of a new family, pray the Our Father together with their brothers and sisters.
Source: Lumen Fidei (Vatican.va)