In the Great Commission Jesus commanded the apostles, “Go, therefore, make disciples of all nations; baptise them in the name of the Father and of the Son and of the Holy Spirit, and teach them to observe all the commands I gave you” (Matthew 28:19-20, NJB). Catechesis is a critical part of making disciples.
Quite early on, the name catechesis was given to the totality of the Church’s efforts to make disciples, to help men believe that Jesus is the Son of God so that believing they might have life in his name, and to educate and instruct them in this life, thus building up the body of Christ. [CCC §4; emphasis in original]
Being a Christian implies the duty to be a disciple, and the task of making them is given to the Church. The advance of Christ’s kingdom is hence entrusted to the Church: as we have seen the Gospel and the Church go together, and as we have seen this connection requires the existence of a visible Church.
A further consequence of this is that we don’t get to decide for ourselves what it means to be a disciple of Christ. He gave the Church the command to make them, and it has a twofold meaning according to Matthew 28. It consists in being properly baptized, and in the observance of Christ’s commands. It is left to the Church to teach us to observe them. It is not left to us to decide what Christ has commanded. It does not follow that the Church is unnecessary just because we have the Word of God in the Bible, and it certainly doesn’t follow that we may judge for ourselves what it is that the Church should teach us. What does follow is that if the Church has been entrusted with the task of making disciples, it falls to us to be teachable, and to receive the Church’s teaching with humility and a willing heart.
Lastly, it becomes clear that how we live and what we believe matters. Jesus has commanded that Christians must be disciples, and that implies that we must be students. The truth matters. So for people to downplay the need for appropriate degrees of certainty when it comes to religious matters is effectively to contradict what Christ has commanded. Mere opinion isn’t sufficient when it comes to religious truth.
Aquinas, Etc. 
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