As is indicated by the title this article is going to illustrate how the Catholic teaching on questions relevant to Christian marriage (e.g. indissolubility, monogamy, prohibition of unlawful sexual intercourse, adultery, homosexuality, lesbianism, Paedophilia, use of contraceptives, etc.) is rooted in the inner life of God himself, i.e. the trinity. For this purpose, an interpretation of the Catholic doctrine on the trinity will be briefly presented by taking the marital act licitly performed in sacramental marriage as point of departure.