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First, the history of the catechism is substantially longer than just 19 years! The catholic catechism dates for hundreds of years - one of the earlier "official" ones was promulgated in 1566.
Catechizing has been a long-standing practice (arguably, God told the Israelites to do that when He told them to write His law on the door posts and tell it to their children).
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Dogmas and doctrines were written down from very early on. Denzinger's Sources of Catholic Dogma is an excellent collection of important dogmas, ordered chronologically.
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I asked this to a presenter in our diocese's catechist training program. She said that the writings of the Council of Trent (1545-1563) were primarily used. I guess that just helps Warren's answer.
Before that, I'd imagine we just looked at stained glass and/or frescoes...