Today’s Catechism sections discuss Baptism as the entry into the Church, which is the Body of Christ. Supporting material comes from the Dogmatic Constitution, “Lumen Gentium”.

Incorporated into the Church, the Body of Christ

1267 Baptism makes us members of the Body of Christ: "Therefore . . . we are members one of another."71 Baptism incorporates us into the Church. From the baptismal fonts is born the one People of God of the New Covenant, which transcends all the natural or human limits of nations, cultures, races, and sexes: "For by one Spirit we were all baptized into one body."72

1268 The baptized have become "living stones" to be "built into a spiritual house, to be a holy priesthood."73 By Baptism they share in the priesthood of Christ, in his prophetic and royal mission. They are "a chosen race, a royal priesthood, a holy nation, God's own people, that [they] may declare the wonderful deeds of him who called [them] out of darkness into his marvelous light."74 Baptism gives a share in the common priesthood of all believers.

1269 Having become a member of the Church, the person baptized belongs no longer to himself, but to him who died and rose for us.75 From now on, he is called to be subject to others, to serve them in the communion of the Church, and to "obey and submit" to the Church's leaders,76 holding them in respect and affection.77 Just as Baptism is the source of responsibilities and duties, the baptized person also enjoys rights within the Church: to receive the sacraments, to be nourished with the Word of God and to be sustained by the other spiritual helps of the Church.78

1270 "Reborn as sons of God, [the baptized] must profess before men the faith they have received from God through the Church" and participate in the apostolic and missionary activity of the People of God.79

The sacramental bond of the unity of Christians

1271 Baptism constitutes the foundation of communion among all Christians, including those who are not yet in full communion with the Catholic Church: "For men who believe in Christ and have been properly baptized are put in some, though imperfect, communion with the Catholic Church. Justified by faith in Baptism, [they] are incorporated into Christ; they therefore have a right to be called Christians, and with good reason are accepted as brothers by the children of the Catholic Church."80 "Baptism therefore constitutes the sacramental bond of unity existing among all who through it are reborn."81

An indelible spiritual mark . . .

1272 Incorporated into Christ by Baptism, the person baptized is configured to Christ. Baptism seals the Christian with the indelible spiritual mark (character) of his belonging to Christ. No sin can erase this mark, even if sin prevents Baptism from bearing the fruits of salvation.82 Given once for all, Baptism cannot be repeated.

1273 Incorporated into the Church by Baptism, the faithful have received the sacramental character that consecrates them for Christian religious worship.83 The baptismal seal enables and commits Christians to serve God by a vital participation in the holy liturgy of the Church and to exercise their baptismal priesthood by the witness of holy lives and practical charity.84

1274 The Holy Spirit has marked us with the seal of the Lord ("Dominicus character") "for the day of redemption."85 "Baptism indeed is the seal of eternal life."86 The faithful Christian who has "kept the seal" until the end, remaining faithful to the demands of his Baptism, will be able to depart this life "marked with the sign of faith,"87 with his baptismal faith, in expectation of the blessed vision of God - the consummation of faith - and in the hope of resurrection.

The Dogmatic Constitution, “Lumen Gentium” discusses the Priesthood of the Faithful.

10. Christ the Lord, High Priest taken from among men,(100) made the new people "a kingdom and priests to God the Father".(101) The baptized, by regeneration and the anointing of the Holy Spirit, are consecrated as a spiritual house and a holy priesthood, in order that through all those works which are those of the Christian man they may offer spiritual sacrifices and proclaim the power of Him who has called them out of darkness into His marvelous light.(102) Therefore all the disciples of Christ, persevering in prayer and praising God,(103) should present themselves as a living sacrifice, holy and pleasing to God.(104) Everywhere on earth they must bear witness to Christ and give an answer to those who seek an account of that hope of eternal life which is in them.(105)

Though they differ from one another in essence and not only in degree, the common priesthood of the faithful and the ministerial or hierarchical priesthood are nonetheless interrelated: each of them in its own special way is a participation in the one priesthood of Christ.(2*) The ministerial priest, by the sacred power he enjoys, teaches and rules the priestly people; acting in the person of Christ, he makes present the Eucharistic sacrifice, and offers it to God in the name of all the people. But the faithful, in virtue of their royal priesthood, join in the offering of the Eucharist.(3*) They likewise exercise that priesthood in receiving the sacraments, in prayer and thanksgiving, in the witness of a holy life, and by self-denial and active charity.

Footnotes

71 ⇒ Eph 4:25.
72 ⇒ 1 Cor 12:13.
73 ⇒ 1 Pet 2:5.
74 ⇒ 1 Pet 2:9.
75 Cf. ⇒ 1 Cor 6:19; ⇒ 2 Cor 5:15.
76 ⇒ Heb 13:17.
77 Cf. ⇒ Eph 5:21; ⇒ 1 Cor 16:15-16; ⇒ 1 Thess 5:12-13; ⇒ Jn 13:12-15.
78 Cf. LG 37; ⇒ CIC, cann. 208 223; CCEO, can. 675:2.
79 LG 11; cf. LG 17; AG 7; 23.
80 UR 3.
81 UR 22 # 2.
82 Cf. ⇒ Rom 8:29; Council of Trent (1547): DS 1609-1619.
83 Cf. LG 11.
84 Cf. LG 10.
85 St. Augustine, Ep. 98, 5: PL 33, 362; ⇒ Eph 4:30; cf. ⇒ 1:13-14; ⇒ 2 Cor 1:21-22.
86 St. Irenaeus, Dem ap. 3: SCh 62, 32.
87 Roman Missal, EP I (Roman Canon) 97.