From Larry Hurtado:
People today sometimes refer to writings “left out” of the NT or refused entry, as if there were many texts vying to be included with the writings that came to be the NT. There were a few that seem to have been considered for a while (e.g., Shepherd of Hermas, a certain “Gospel of Peter”, maybe 1 Clement). But it is unlikely that the authors of Gospel of Thomas, Gospel of Truth, Gospel of Philip, or the several apocryphal acts ever wanted their texts to be part of a NT collection. The Gospel of Thomas, for example, reflects an intense disdain for ordinary Christians, and claims to deliver a unique and secret body of teaching of which only certain believers are worthy. It’s elitist to the core, so it’s unlikely that those responsible for it ever wanted to have it treated as one text/voice among others. (As as to the mainstream Christian rejection of the stance reflected in Gospel of Thomas, I’m reminded of the quip from the American comic, Jerry Seinfeld: “Sometimes the road less taken is less taken for a reason!”)
So, historically, the NT represents the inclusiveness that characterized earliest “proto-orthodoxy” (as I’ve noted in my chapter on “Proto-Orthodox Devotion” in Lord Jesus Christ, 563-648). For Christians thinking about diversity among themselves today, there just might something there worth pondering.