What exactly is the Didache? Is it reliable teaching and is it biblical?
The Bible says of itself that “the word of God is living and active, sharper than any two-edged sword, piercing to the division of soul and of spirit, of joints and of marrow, and discerning the thoughts and intentions of the heart” (Heb 4:12), and this two-edged sword cuts both ways; for the one giving it out but also by the one who hears it, but it cuts in order to heal and bring eternal life. The Apostle Paul states a truth for the ages in writing that “All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness” (2nd Tim 3:16), so the man or woman “of God may be complete, equipped for every good work” (2nd Tim 3:17). Jesus Himself once quoted Scripture, saying “Have you never read in the Scriptures: ‘The stone that the builders rejected has become the cornerstone; this was the Lord’s doing, and it is marvelous in our eyes’” (Matt 21:42)? The Greek word Paul used for the word “Scripture” is “graphē” from which we get our word, autograph, and that’s pretty much what the Scriptures are; the autographs or writings of the men who wrote them, but these writings themselves are breathed out by God through His Spirit, not by men’s reasoning’s. God used these men as they recorded what they are told. How that worked, I have no clue, but think of it this way. My computer is like an autograph for me but I am the inspiration in writing this. My computer can’t be credited with writing this because it can’t create a document on its own. It needs my help, so when people criticize the Bible and say “it’s just the words of man” are only suppressing the truth that the Word of God  comes straight from the mouth of God. Not one word of God has ever failed from Scripture, nor will one ever.
Some of the greatest events in church history have been recorded by some of the greatest historians in history, and these are not just religious historians, but secular historians who had no special interest in writing down what they saw and heard, and some of these historians have written down some of the richest church histories you can find anywhere, so they are certainly worth exploring. These include the Jewish historian, Titus Flavius Josephus who wrote a rich history of the Jews, up to his present day, when he died in AD 100 at age 63. In his “Antiquities of the Jews,” he also recorded many of the events which took place at the turn of the century, dedicating a good portion of his writings to Jesus Christ and the church in the first century. This book goes into some detail about some of Jesus Christ’s teachings and activities in the first century, but also about the first century church. Then, there is a book called, Josephus on Jesus. This is a very interesting book because much of Josephus’ writings about Jesus are condensed into one book, and it is a good primary (eye witnesses) source for reading about Jesus, but also includes many secondary sources (accounts of other witnesses) of Jesus’ and His disciples, but also includes some of the earliest Christians found in the New Testament like Paul, Peter, and James.
Luke the Physician and Historian
The Gospel of Luke and the Book of Acts, which were both written by Luke, are considered even by contemporary historians to be a very accurate history. Luke’s account of Jesus’ life and death, and first century life in Judea and the Roman Empire, are full of dates, names, and places that have since been verified. This is why Luke is considered to be one of the best early historians of the time, as far as his writings are concerned, and amazingly, Luke and Josephus’ writings are remarkably close to one another, giving finite details and specific dates, times, places, names, and specific events in the Bible (like the crucifixion, the persecution, etc.). This included the Person and work of Jesus Christ, so Luke, Josephus, and many other historians, have recorded the accounts of the lives and events of the apostles and the first century church, and go into great detail in writing about the severe persecution the early church faced. Now, what about the Didache? Is it reliable? Is it truthful? Can we consider it as inspired, or simply, inspiring.
The Didache simply means “teaching” but is better known as “The Teaching of the Twelve Apostles.” The Didache is considered a gathering of the teaching of the disciples or apostles that Jesus Christ selected, and is anonymously authored, and is directed toward the Gentiles and concerns matters like life and death, church order, fasting, baptism, prayer, and a host of other teachings that Jesus passed onto the apostles. Certainly some of the teachings are good, and even biblical, like in chapter 2 where it says, “Thy speech shall not be false or empty, but concerned with action” (2:5) which is found in principle in James 2:14-17, and “but be thou meek, for the meek shall inherit the earth” (3:7), where Jesus said, “Blessed are the meek, for they shall inherit the earth” (Matt 5:5), but parts of it are troubling and don’t sound biblical at all. One such verse (and there are others) is where it says, “And let the apostle when departing take nothing but bread until he arrive at his resting-place; but if he ask for money, he is a false prophet ” (11:6). I once had a man use this quote to say that a pastor should never ask for money and not draw a salary. I thought, he was making a false conclusion from a man-made document from the writings of men and trying to make it sound like it was from God’s Word. The Didache is clearly not inspired by the Holy Spirit, and even if it’s inspiring, is not inspired. I don’t think anyone would argue with that, but it’s not a good book to quote when trying to correct another believer. It might make a good study, but why not abide in the Word of God, which you know is God speaking and has real power (Rom 1:16; 1 Cor 1:18). Parts of the Didache appear to conflict with one another, because we’re told that if an apostle (or a pastor?) asks for money, they are “a false prophet,” but then it says, “in like manner when thou openest a jar of wine or of oil, take the firstfruits and give it to the prophets” (13:6). What’s the prophet supposed to do; if he turns it down, he makes the others who give to him be disobedient but if he accepts it, he’s called a “false prophet.” What if a missionary is asking for money from his church to support a missionary trip? Surely, that doesn’t make them a false prophet, does it?
I hope you can see by now why the Didache is an interesting book and it does contain some truth, but we know that the Word of God is without error and is always true. It never conflicts with itself, unlike the Didache did a few times. God is not a God of chaos, but One of order (1st Cor 14:33), and what God says in His Word comes straight from the breath of God. The Didache? Not so much.
Read about these other historical books: What Are the Apocrypha Books? 
Resource – Scripture quotations are from The Holy Bible, English Standard Version® (ESV®), copyright © 2001 by Crossway, a publishing ministry of Good News Publishers. Used by permission. All rights reserved.