In a justly famous passage in Adversus Haereses, St Irenaeus of Lyons describes both the proper use (by the Church) and the misuse (by heretics) of the Scriptures using the metaphor of a mosaic image of a great king. When Scripture is misused, it is as if the mosaic of the king is taken to pieces and the gems re-arranged to show an entirely different image. If one does not know what the mosaic was originally intended to portray, one would not know that anything is amiss. (The passage is a bit long to quote here in full, but those who are unfamiliar with it can read it here.)
In the following chapter, St Irenaeus goes on to give a summary of the rule of faith itself, and the contents of that rule will not be a surprise to any orthodox Christian. But what interests me in the current passage is a passing reference to where the rule comes from. Discussing how a Christian may recognize the misuse of Scripture by false teachers, St Irenaeus writes:
Anyone who keeps unswervingly in himself the canon of truth received through baptism will recognize the names and sayings and parables from the Scriptures, but this blasphemous hypothesis of theirs [viz. of the heretics] he will not recognize. For if he recognizes the jewels, he will not accept the fox as the image of the king. He will restore each one of the passages to its proper order, and having fit it into the body of the truth, he will lay bare their fabrication and show that it is without support.
St Irenaeus tells us that the canon of truth, the very key to the proper interpretation of the Holy Scriptures, is given to us in baptism. Not at seminary, nor at ordination, nor at election or installation as district president, bishop, patriarch, or pope; but in baptism.
Does this mean that the key to the Scriptures is somehow magically transmitted in the baptismal liturgy? Indeed not. Baptism is not an isolated ceremony, but the central event in a complex process of Christian initiation. That process includes the catechesis of the new believer before baptism, the gradual inclusion of the catechumen into the worshipping community, the baptism itself, the gift of the Holy Spirit through laying-on of hands or chrism, and the mystagogical catechesis that takes place after baptism. I would contend that when St Irenaeus tells us that the canon of truth is given to us through baptism, he is referring not just to the washing, but to that entire process by which which the Gospel is handed over (παρεδωκα) to each new Christian. Christ Himself set this context for baptism, commanding us not simply to baptize, but to make disciples ... teaching them to observe all things whatsoever I have commanded you.
With the widespread practice of infant baptism, we've somewhat inverted the process: baptism comes first and the catechesis and inclusion into the worshipping community comes later, over a number of years. The same tradition is delivered even in this inverted order, but it makes it hard for us modern Christians to recognize that we receive the Apostolic Tradition, the κανόνα τῆς ἀληθείας, "through baptism."