The authority of sacred Scripture should be authoritative enough, but in case Scripture fails your logic, the Church also says that Truth is identified and lived through and with sacred Tradition and the Magisterium. More clearly: the believing Catholic holds that Truth is revealed in three inter-related pillars: sacred Scripture, Tradition and the Magisterium. One can't have one of the pillars without the others. To do so would mean that you are Protestant.
Concerning many in the Church (catechist, laity and ordained) is the lack of understanding of who Jesus is, and His place in our lives today. Many Catholics are functional agnostics; they have no concept of who Jesus is, why He is important, and what he has to do with the Church. No surprise to me since I contend that many of the ordained can't adequately explain matters of Christology or Sotierology (the study of Chist and the study of salvation respectively) as is evidence in their praying the Mass, preaching, their practice of the sacraments in their own lives, and their teaching in other venues. I would also contend that many of the ecclesial problems we face today are the direct result of not really knowing who Jesus is, and how to conform ourselves to His Way. The Pslams, as one example, tell us to seek the face of the Lord (the Christian would understand this to mean, seek the face of Christ). As one consequence of not knowing Jesus is the denial that we are already saved --that salvation has already happened, that the hundredfold promised by the Lord is already fulfilled. Do you know that you are saved? Do act as though you are saved or are still persisting in your sinful ways?
In the recent English edition of the L'Osservatore Romano (7 April 2010), Lucetta Scardaffia's article "The Shroud and secularization" makes a few good points to think about when asking the questions about who Jesus is and what we face today:
"It seems incredible, but many young people do not even know that Jesus existed historically: in various countries, including Italy, today the history of Christianity no longer forms part of the school curriculum, and this leads to an ignorance that is also the result of tendencies geared to make Christianity a religion like others, with no specificity, hence leading to considering Christ as a mythical being, almost as if he were a Greek, Roman or Orienal divinity.
In international milieus -- even in organizations such as the United Nations-- propaganda for a mistaken concept of multiculturalism has been spreading for decades: this is proposed as a panacea for every conflict, providing that all religions be considered absolutely equal, in other words that each waives all claim to truth.
Remembering that Christianity is born from the existence in history of a man, Jesus, who said he was the Son of God, is an obstacle to the seemingly irenic fabrication because it highlights the difference of Christianity in comparison with other religions. A God who becomes incarnate to save human beings, in fact is an absolute unicum [absolutely unique], difficult to standardize.
The history of the birth of the individual is also interwoven with the history of the theological and sacramental significance of the Body of Christ --with the history of the sacrifice of the altar, for which the Body becomes a monstrance.