| Term | Definition |
| mysterion | The Greek word for mystery, which applies to what Paul called the mysterious wisdom of God. |
| sacraments of initiation | Three sacraments that represent an official introduction to the Church. They are baptism, confirmation, and Eucharist. The three sacraments are meant to serve as a progressive incorporation into the Church. |
| intercommunion | The question of Eucharistic hospitality, or who is included in the Eucharist. Many rules guide the taking of communion in other Churches, even during the last rites. |
| catechumen | Each candidate for the "catechumenate" or the incorporation used by the Church in its early years. |
| RCIA | The Rite of Christian Initiation of Adults, a new rite that includes five in-depth stages for non-Catholic adults to become Catholics. |
| harmartia | In Greek, literally "to miss the mark," or "to go wrong." Paul uses the word for sin, and claims that God's relationship with his creation is disordered when harmartia occurs. |
| Pelagius | A British ascetic who believed that after baptism, Christians had the ability to live a life of perfection through free will, thereby discounting the need for a savior (Jesus). |
| Imago Dei | The image, likeness, and intelligence of God reflected in humanity. |
| humane vitae | An encyclical released in 1968 by Pope Paul Vi rejecting modern methods of contraception. |
| evangelium vitae | An encyclical released in 1995 by Pope John Paul II reaffirming the Church's unwavering commitment to the dignity of human life. |