Nothing in your quoted text says that Baptism is the only necessary thing for salvation. Actually, the Catechism disagrees with you on this point.
Baptism is necessary for the unbaptized.
1257 The Lord himself affirms that Baptism is necessary for salvation.59 He also commands his disciples to proclaim the Gospel to all nations and to baptize them.60Baptism is necessary for salvation for those to whom the Gospel has been proclaimed and who have had the possibility of asking for this sacrament.
All the Sacraments are necessary for believers.
1129 The Church affirms that for believers the sacraments of the New Covenant are necessary for salvation.51 “Sacramental grace” is the grace of the Holy Spirit, given by Christ and proper to each sacrament. the Spirit heals and transforms those who receive him by conforming them to the Son of God. the fruit of the sacramental life is that the Spirit of adoption makes the faithful partakers in the divine nature52 by uniting them in a living union with the only Son, the Saviour.
Well, the above demonstrates that you are in error, first of all, but it does not prove my position. So, what about all the rest? I will try to make my case.
Baptism as a Sacrament effects Grace. Indeed, salvation is found in Grace and nothing else; specifically, salvation is found the Graces of forgiveness and new life. As state by the Catechism, these are the two principle effects of Baptism:
1262 The different effects of Baptism are signified by the perceptible elements of the sacramental rite. Immersion in water symbolizes not only death and purification, but also regeneration and renewal. Thus the two principal effects are purification from sins and new birth in the Holy Spirit.64
But Grace is only as effective in a person as the person’s disposition to it.
1128 This is the meaning of the Church’s affirmation49 that the sacraments act ex opere operato (literally: “by the very fact of the action’s being performed”), i.e., by virtue of the saving work of Christ, accomplished once for all. It follows that "the sacrament is not wrought by the righteousness of either the celebrant or the recipient, but by the power of God."50 From the moment that a sacrament is celebrated in accordance with the intention of the Church, the power of Christ and his Spirit acts in and through it, independently of the personal holiness of the minister. Nevertheless, the fruits of the sacraments also depend on the disposition of the one who receives them.
Thus, for example, if one is not disposed to receive the grace of forgiveness and new life, to enter into the life of the Church, etc, then Baptism will not effect in that person the Graces it ought. In other words, you can’t baptize someone who doesn’t want to be baptized.
Likewise, if a believer is not disposed to the Graces of the Sacraments of the New Covenant, then the fruits of those Graces are stifled. And what are the fruits of the Sacraments?
As I quoted above: “the fruit of the sacramental life is that the Spirit of adoption makes the faithful partakers in the divine nature52 by uniting them in a living union with the only Son, the Saviour” We are saved through the fruits of the Sacraments, in that we become one with the Son, the Saviour. That is our new life.
But if a positive disposition is necessary in order for this fruit to be effective in us, then that which produces within us a positive disposition to such graces must also, therefore, be necessary. Baptism, Confirmation and Holy Orders produce this within us as an indelible seal:
1121 The three sacraments of Baptism, Confirmation, and Holy Orders confer, in addition to grace, a sacramental character or “seal” by which the Christian shares in Christ’s priesthood and is made a member of the Church according to different states and functions. This configuration to Christ and to the Church, brought about by the Spirit, is indelible,40 it remains for ever in the Christian as a positive disposition for grace, a promise and guarantee of divine protection, and as a vocation to divine worship and to the service of the Church. Therefore these sacraments can never be repeated.
But guess what, Sacramental and pious devotions also do this (from Chapter 4, Article 1 Sacramentals):
1670 Sacramentals do not confer the grace of the Holy Spirit in the way that the sacraments do, but by the Church’s prayer, they prepare us to receive grace and dispose us to cooperate with it. "For well-disposed members of the faithful, the liturgy of the sacraments and sacramentals sanctifies almost every event of their lives with the divine grace which flows from the Paschal mystery of the Passion, Death, and Resurrection of Christ. From this source all sacraments and sacramentals draw their power.