Fathers of the Church blog has a good introduction to the Didache. This first century document brings into focus the Church at near apostolic times.
Twenty-first century Christians tend to romanticize those founding years of the Church as a golden age of unity, when believers absorbed sound doctrine by osmosis, and when Christians couldn’t help but love one another, and bless their persecutors, and feed the poor.
But that’s not how it was. Early on, the Church faced serious threats from self-proclaimed Christians who denied, for example, that the eternal Word truly became flesh (see 1 Jn 4:2 and 2 Jn 1:7). They also denied the reality of the Eucharist and the necessity of the Church. Quite early in the game, there were even some teachers who held that revelation was a private affair between God and the individual believer. They spun wildly creative religious systems (see 1 Tim 1:4) and gave a green-light to unbridled lust (see Jd 7). To legitimize their “revelations,” such heretics often attributed oracles to the apostles (see Gal 1:7 and 2 Thess 2:2).
Amid this confusion came order and orthodoxy in the Didache. It is, perhaps, the earliest ancestor of today’s Catechism of the Catholic Church. And, indeed, the new Catechism quotes that first one several times
The Didache is evidence that the beliefs and practices central to our modern era Catholic Faith have been part of our Faith since the beginnings of the Church. When modern heresies arise (often just echoing previous ones) we need to be prepared to answer with confidence. Knowledge of the early Church is a good start.