Lex orandi, lex credendi. The law of prayer is the law of belief. This means that how we pray will inform what we believe -- if we want to believe as all Catholics have believed through all the ages, we must also pray as Catholics have always prayed. This holds especially true in the Sacred Liturgy: It is impossible to have a radical change in the form of liturgical worship without risking the integrity of the faith.
It is astonishing to realize that the Mass was basically the same from the end of the early persecutions when we were first permitted to have public worship until the time of the Second Vatican Council (roughly, for 1,600 years from 360 to 1960). However, the common way that Mass is celebrated since Vatican II bears little resemblance to the form of worship which was common to all the saints and indeed all Catholics for all time.
The Second Vatican Council did not intend this extreme change in the form of worship. Indeed, Popes St John Paul II, Benedict XVI, and Francis have all been guiding the Church to realize that there should be continuity between how the Mass was celebrated before and after Vatican II. When we attend the Extraordinary Form of the Mass (aka Traditional Latin Mass) and then attend the Novus Ordo Mass of Vatican II, we should be struck not by how different but rather by how similar they are. We should be able to pray in basically the same way in the New Mass as we do whenever we attend the Traditional Mass.
If we want to have the same faith of the saints of all the ages, we need to pray in the same way as the saints of all ages have prayed - and this holds most especially for the Holy Mass. Which is to say: It's not good enough to be reverent and conservative, we must be traditional when participating in the Mass of Vatican II.
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