During the mass, the ringing of some bells is heard twice. The first is when invoking the Holy Spirit over the offerings and the second is when the priest is consecrating them (GIRM 150), why are said bells ringing? This tradition dates back from before the Second Vatican Council, when the priest celebrated Mass with his back to the congregation, in a low voice and in Latin, not in the language of the people as it is today. This usually caused the assembly to be distracted, in such a way that, to highlight these extremely important moments, it was established that the bells would be rung. After the Second Vatican Council, this tradition was maintained to continue marking the peak moment of consecration, in which bread and wine are transubstantiated into the body and blood of our Lord Jesus Christ.
The ringing of the bells centers our attention on the fact that we are before our Lord Jesus Christ, truly, really and substantially present in the Blessed Sacrament; and it is the moment in which some people out of devotion repeat the words of Saint Thomas, “My Lord and my God” (John 20:28). It is worth mentioning that these words should be said rather, inside our hearts; since the consecration, being part of the actions of the one who presides over the mass, must be pronounced with a clear and loud voice, and that everyone listens carefully. Therefore, while the priest consecrates the offerings, there should be no singing, prayers, or music (GIRM 32). The act of elevating the body and blood of Christ is part of the consecration.
Let us always keep attention during mass, and may the sound of the bells remind us that it is Jesus Christ himself who is present in the Holy Sacrament of the Eucharist, and we must receive Him in communion in order to enjoy eternal life with Him in the Kingdom of the Heavens.