How can a newborn baby be a sinner so that it has to be baptised?
At the very beginning of the Church there is naturally only the baptism of adults […] The baptism of children can only become an issue or accepted practice in the second generation. There are no direct references for it in the New Testament itself. However, the New Testament repeatedly speaks of the practice of baptising a whole household, i.e. whole families including all their servants. (cf. Acts 16:15, 33-34; 18.8; 1 Cor 1:16). It is possible that this also included children. The first clear and specific references to infant baptism date back to the 2nd century. The practice of infant baptism has therefore existed for a very long time in both the churches of the East and the West. Several popes and synods, and in particular the Council of Trent (cf. DS 1514; 1626-1627; NR 356; 544-545), confirmed and defended this doctrine and practice […]
There are three main reasons that justify the practice of infant baptism.
1. Being a Christian through Baptism is a free, unmerited grace through which God acts prior to all our acting and surrounds our lives from their beginning (cf. 1 Jn 4:10,19; Tit 3:5), and which we all need from our earliest beginning because of original sin. This grace that comes before all doing and any merit is expressed especially clearly in infant baptism. The Church and the Christian parents would therefore deny the child a very important good if they did not give him the sacrament of baptism soon after his birth.
2. Faith always points to and depends on the communion of believers. Infant baptism clearly symbolises this dependence and inclusion in the overall community without which the child literally could not survive. Through his parents and godparents the child is thus taken up in the communion of all believers who are responsible for this child before God and before the world. That is one of the reasons a child may only be baptised if the parents or relatives guarantee his subsequent Christian upbringing. Where this guarantee is not given, baptism is wisely to be postponed.
3. Faith is not an event in time but a process of growing. Growing into Christ and into faith in him is a lifelong process for the baptised Christian. The New Testament not only knows of the movement that leads from faith to baptism and that finds in baptism its most complete form of embodiment (cf. Acts 8,12-13; 18,8; 10,47 u. a.). There is also the converse movement, whereby the baptised are reminded of their baptism and led deeper and deeper into the baptismal reality (cf. Rom 6:3-4, 1 Cor 6:9-11; Eph 5:8-9; 1 Pt 2:1-5). After all, baptism is not only a sign of faith but also the source of its power; it is the sacrament of enlightenment. As such, it is the beginning of a path and a life-long growing in faith.
From this reasoning arises the necessity of a renewed system of adult preparation for baptism (Taufpastoral). Fundamentally, all pastoral care is aimed at leading people towards baptism and the development of the new life inherent in baptism in individual Christians and in the parish. The adult preparation for baptism in the narrower sense includes the adult catechumenate, and in the case of infant baptism, the baptismal preparations with the parents and godparents, pastoral care of wedding couples and newly married couples, and leading the whole congregation into a shared responsibility for the Christian upbringing of the children. Of particular importance in this context is the renewal of the congregational catechesis, through which growing children shall be introduced into the faith and the life of the Church (cf. Gem. Synode, Schwerpunkte heutiger Sakramentenpastoral 2-3). The sacrament of this growth is confirmation. (Katholischer Erwachsenen-Katechismus, Vol. 1, p. 337-339)