The "Catechism of the Catholic Church" teaches that "the flesh is the hinge of salvation". The "Catechism" indicates that sexual relationships in marriage is "a way of imitating in the flesh the Creator's generosity and fecundity" and lists fornication as one of the "offenses against chastity", calling it "an intrinsically and gravely disordered action" because "use of the sexual faculty, for whatever reason, outside of marriage is essentially contrary to its purpose". The "conjugal act" aims "at a deeply personal unity, a unity that, beyond union in one flesh, leads to forming one heart and soul" since the marriage bond is to be a sign of the love between God and humanity. Pope John Paul II's first major teaching was on the theology of the body, presented in a series of lectures by the same name. Over the course of five years he elucidated a vision of sex that was not only positive and affirming but was about redemption, not condemnation. He taught that by understanding God's plan for physical love we could understand "the meaning of the whole of existence, the meaning of life." He taught that human beings were created by a loving God for a purpose: to be loving persons who freely choose to love, to give themselves as persons who express their self-giving through their bodies. Thus, sexual intercourse between husband and wife is a symbol of their total mutual self-donation. For John Paul II, "The body, and it alone, is capable of making visible what is invisible: the spiritual and divine." He says there is no other more perfect image of the unity and communion of God in mutual love than the sexual act of a married couple, whereby they give themselves in a total way – exclusively to one another, and up to the end of their lives, and in a fruitfully generous way by participating in the creation of new human beings. Through this perspective, he understands the immorality of extra-marital sex. It falsifies the language of the human body, a language of total love worthy of persons by using the body for selfish ends, thus treating persons as things and objects, rather than dealing with embodied persons with the reverence and love that incarnate spirits deserve. John Paul II stresses that there is great beauty in sexual love when done in harmony with the human values of freely chosen total commitment and self-giving. For him, this sexual love is a form of worship, an experience of the sacred. Roman Catholics believe that masturbation is a sin.