The Codex Sinaiticus contains the "Epistle of Barnabas" in full, but it is not in the agreed New Testament canon, and it was included in the list of "Disputed Books" by Eusebius.  However, parts of it are cited by several patristic writers.

The writer claims that God's covenant with Israel was cancelled when Moses broke the tablets on coming down from the mountain to find the Israelites worshipping a golden calf.   The writer quotes from several Old Testament writers attacking sacrifices, including Isaiah and the writer of the Psalms.

The writer uses eschatological language, believing the world to be under the control of "The Active One" -- "The Black One"  -- who even has power to eject people from the kingdom of heaven.

The thrust of the writer's argument is similar to that of Amos, seeing social responsibility as more important than liturgical observance.