The Didache has been known in an extended Ethiopic version, called the Didascalia for a very long time -- it's actually part of the extended New Testament canon of the Ethiopian Orthodox Church. It was discovered in its Greek original, though, in 1873, in the library of an Orthodox monastery by the Orthodox Metropolitan of Nicomedia Philotheos Bryennios.
The Didache (Greek, meaning "teaching") is a church order manual. Some of the early Fathers considered it Scriptural, but it was eventually excluded from the New Testament largely because it was unnecessary to include a manual of church order in Sacred Writ.
So let's look at the Faith as it was believed and lived by the early Christians who lived even before Scripture, and compare it on a few points with the Faith of the Orthodox Church today.
"But concerning baptism, thus shall ye baptize. Having first recited all these things, baptize in the name of the Father and of the Son and of the Holy Spirit in living [running] water. But if thou hast not living water, then baptize in other water; and if thou art not able in cold, then in warm. But if thou hast neither, then pour water on the head thrice in the name of the Father and of the Son and of the Holy Spirit. But before the baptism let him that baptizeth and him that is baptized fast, and any others also who are able; and thou shalt order him that is baptized to fast a day or two before." - Didache, 7:1-7The early Christian practices of Baptism via triple immersion and fasting before Baptism are still preserved in the Orthodox Church today.
"And let not your fastings be with the hypocrites [Jews], for they fast on the second [Monday] and the fifth [Thursday] day of the week; but do ye keep your fast on the fourth [Wednesday] and on the preparation [the sixth -- Friday] day." - Didache, 8:1-2The early Christian practice of fasting on Wednesdays and Fridays is still preserved in the Orthodox Church today.
On the Eucharist:
"But let no one eat or drink of this eucharistic thanksgiving, but they that have been baptized into the name of the Lord; for concerning this also the Lord hath said: Give not that which is holy to the dogs." - Didache, 9:10-12The early Christian practice of closed Communion (that is, Communion only for Baptized members of the Church) is still preserved in the Orthodox Church today.
As you can see, the Didache preserves for us a record of how the earliest Christians lived, Christians who lived even before the writings of the New Testament were put to pen and paper. Sola Scriptura would have been completely impossible for them -- and yet they were a thriving Christian community and one whose Faith is still preserved and observed in the Holy Tradition of the Orthodox Church today, even in small details like triple immersion Baptism and fasting on Fridays. The community which produced the Didache was Apostolic in the most literal sense of the word and this document is a demonstration of the Apostolic nature of the Orthodox Church's Holy Traditions -- Traditions which may not be explicitly laid out in Scripture, in some cases, but which in many instances pre-date Scripture!
If you'd like to read the Didache for yourself and make your own comparisons with the various Christian groups today, you can check it out online here or buy a hardbound version of The Didache here.