II. THE INCARNATION
Belief in the true Incarnation of the Son of God is the distinctive sign of Christian faith: "By this you know the Spirit of God: every spirit which confesses that Jesus Christ has come in the flesh is of God." Such is the joyous conviction of the Church from her beginning whenever she sings "the mystery of our religion": "He was manifested in the flesh."
Source: Catechism of the Catholic Church (Vatican.va)