I recommend this excellent resource compiled from early church teachings.
Early Christian Commentary of the Sermon on the Mount
Authored by Elliott Nesch
What the world needs today is not a new definition of Christianity, but a present-day demonstration of Christianity. Rather than re-define Christianity to accommodate our present generation, we must re-discover Christianity from the apostolic generation. In this regard, the early Christian writings are a helpful tool.
During the first three centuries of Christian history, many books, commentaries, letters and sermons were being circulated among the ancient Church. Many of these writings have survived until our own time. Capturing the true essence of apostolic Christianity, the early disciples provide an extremely valuable post-New Testament history and commentary on the New Testament Scriptures. Today, their writings provide us a record of primitive Christianity during the time period directly following Jesus Christ and the Apostles.
The beliefs and practices found within modern Christendom most often part ways with early Christianity when it comes to interpretation of the Lord's sayings in the Sermon on the Mount and participation in the divine life. Moreover, there are men of education and expertise, esteemed widely as safe and sound expositors of Scripture, who make it their business to hinder the disciples of Christ who would go up the mountain where Christ's own words are to be heard.
A substantial number of the early Christians-Barnabas, Ignatius, Polycarp, Justin Martyr, Apollonius, Hippolytus, Cyprian, Methodius, and Origen-died as martyrs. Like the apostles, the early Christians were willing to die for their beliefs. Therefore we ought to seriously consider what they have to say to us today, especially when it comes to their understanding of history's greatest sermon ever preached. May the reader be edified and blessed as we ascend the mountain together with the early Christians to hear the words of eternal life from our Lord Jesus Christ. . . .
SI Moderator - Greg Gordon