Is this why some mainline Protestants are OK with quoting from the Gospel of Thomas?
Are you saying that basically magisterial Protestants have an indefinite canon?
Is this why debate about the authorships of certain books is so critical for some?
Note that I'm speaking of mainline churches. "Magisterial" refers to Lutherans and Reformed. I wouldn't want to be understood as speaking for the conservative parts of those traditions.
In worship we use the 66 book canon. However when exegetes and theologians work, they look more widely. So I don't think our churches would say the canon is indefinite. But you need to look at how it's used. Luther certainly had books he didn't think rated as high as others (e.g. James), and Calvin didn't comment on or refer to the Revelation. But used some books more heavily than others.
However the mainline today goes further than that, based on critical judgements of each book: authorship, date, purpose, theological outlook, etc.
Some writers on this issue refer to a "stepped canon." They point out that Jews saw the Torah as key, with a kind of continuum of books going out from there, with the Prophets having a bit higher status than the Writings, and then a bunch of D-C books and other books that were used.
Most PCUSA preachers use the lectionary. You can't make a lectionary without a canon. But when you get to serious theology, I think the boundaries are wider and grayer, with people making assessment of each book individually.
Thomas. Some scholars claimed that this gave, or sometimes gave, earlier forms of Jesus' teachings. The "Scholar's Press" NT treated it in parallel with the 4 canonical Gospels. Outside that fairly small set of scholars, I haven't seen much support for this. I personally don't think it gives us much information about Jesus' teachings that aren't there in the canonical Gospels, so I don't tend to use it. The commentaries I normally use don't refer to it very often.
Yes, authorship matters. If you assess books individually, it matters to me whether 2 Peter is a late book by someone other than Peter or Peter's actual work. But it's not a question of whether I redefine the canon to omit 2 Peter. Rather, it's a matter of how I would use it.