The Letter of Barnabas was not written by Paul's disciple, as has been commonly presumed at different times in the past, but was either a forgery or written by some other Barnabas. Arthur Coxe, editor of Ante-Nicene Fathers, wrote, "The ancient writers who refer to this Epistle unanimously attribute it to Barnabas the Levite, of Cyprus, who held such an honourable place in the infant Church."
Whoever wrote it, I do not believe it to be authoritative, and I am not alone.
Coxe continues, "On perusing the Epistle, the reader will be in circumstances to judge of this matter for himself. He will be led to consider whether the spirit and tone of the writing, as so decidedly opposed to all respect for Judaism-the numerous inaccuracies which it contains with respect to Mosaic enactments and observances-the absurd and trifling interpretations of Scripture which it suggests-and the many silly vaunts of superior knowledge in which its writer indulges-can possibly comport with its ascription to the fellow-labourer of St. Paul. When it is remembered that no one ascribes the Epistle to the apostolic Barnabas till the times of Clement of Alexandria, and that it is ranked by Eusebius among the "spurious" writings, which, however much known and read in the Church, were never regarded as authoritative, little doubt can remain that the external evidence is of itself weak, and should not make us hesitate for a moment in refusing to ascribe this writing to Barnabas the Apostle."
All of the Apostles were highly respectful of the Law, if not all of Jewish tradition, which had come to supplant the Law. This Barnabas, on the other hand, seems to be at pains to discredit the Law and to rebuild the wall of separation between Jews and Gentiles that Yeshua gave up his life to tear down.
Barnabas is not completely devoid of truth, but it contains enough error that it should be read with caution and should certainly not be the basis for any doctrine. For example, while the dietary restrictions given in the Books of the Law may well have a secondary meaning, they were not inventions of Moses nor were they meant not to be literally obeyed. Whatever hidden meaning the restrictions may have, the writer did not know them. This is obvious from his bizarre characterizations of the animals involved. (He says the hyena is unclean because it can change its gender. It cannot, but some coral reef fish, which are clean, can change gender.)
I believe the misattribution of this letter to Barnabas, the associate of Paul, contributed to the proliferation of many false doctrines in the early Christian church. In this work, as well as in The Shepherd of Hermas, the Books of Adam and Eve, the Nag Hammadi library, and numerous works which no longer exist, lie the roots of the Roman church's errors concerning sex and marriage.