The Didache is a magnificent little document that is likely the oldest non-biblical Christian text, dating to about A.D. 70-90. I came across one line that is a pure gold nugget of Catholic practice and doctrine:
On the Lord's own day, assemble in common to break bread and offer thanks (Greek: "to Eucharist"), but first confess your sins so that your sacrifice may be pure." Didache, 14.
Here we have an imperative for the Sunday obligation - to come together for the Eucharist. Next, there is an imperative that Christians "confess your sins". Thirdly, the act of "breaking the bread" is referred to as a "sacrifice". In all three regards, typical Protestant practice and theology diverges from this very early witness to Christian piety. We find that the early Apostolic Church already practiced an obligatory Sunday Eucharist, confession of sins, and belief in the sacrificial aspect of the Eucharist.