One could do it on the basis of received doctrine: “My church teaches that X is the canon of the NT; this agrees with Athanasius; therefore, Athanasius was right about the canon. My church teaches Y about communion; this disagrees with Athanasius; therefore, Athanasius was wrong about communion.”
Further, one does not need to agree with both A and B: “My church teaches that X is the canon of the NT; this agrees with Athanasius; therefore, Athanasius was right about the canon. My church teaches Y about the Eucharist; this neither agrees nor disagrees with Athanasius; therefore, Athanasius’ view of communion may or may not have been right.”
Indeed, that can also apply on the basis of personal study: “My understanding is that X is the canon of the NT; this agrees with Athanasius; therefore, Athanasius was right about the canon. My understanding is that the Eucharist is Y; this neither agrees nor disagrees with Athanasius; therefore, Athanasius’ view of communion may or may not have been right.”
That is exactly my point though. In each of those instances the person makes a determination as to whether Athanasius was right or wrong on a particular point based on what the person already believes to be true, whether that belief is based upon what his church teaches or on his own personal study. Therefore, when the question becomes “how do you know that the 27 book NT is correct?,” and the person answers “because Athanasius listed the same 27 book NT canon way back in 367 AD,” his reasoning becomes circular because the reason he believes Athanasius was correct on the canon is because Athanasius agrees with what he already believes. It’s begging the question.
As noted before, the two ideas (the canon and the Eucharist) are parallel, not interdependent, and assent to one view does not logically entail assent to the other.
I understand that the ideas themselves (the canon and the Eucharist) are not interdependent; however, if one is using Athanasius–or the Fathers in general–as the authority for determining the canon, then what basis does the person have to reject them as authority for determining Eucharistic belief, other than because they do not agree with what the person already believes?
You state that it is not logically inconsistent, but actually it is in this instance, precisely because the Fathers are being used as a basis for authority. It is logically inconsistent to appeal to the Early Church Fathers as authority for the basis of a belief if one only appeals to them as authority in those instances in which the Fathers agree with what the person already believes.
We can illustrate this logically as follows:
IF A = B, THEN A is TRUE; IF A ≠ B, THEN A is FALSE.
If one is going to appeal to the Fathers as authority, then logically what
should happen is the following:
Scenario 1: The NT Canon
A: I (and/or my church) believe the New Testament canon consists of the 27 NT books.
B: Athanasius (and/or the Fathers) believed the New Testament canon consited of the 27 NT books.
THEREFORE, my (and/or my church’s) belief in the 27 book NT canon is TRUE.
Scenario 2: The Eucharist
A: I (and/or my church) do not believe in the actual, substantial Real Presense of Christ in the Eucharist.
B: Athanasius (and/or the Fathers) believed in the actual, substantial Real Presense of Christ in the Eucharist.
THEREFORE, my (and/or my church’s) rejection of the actual, substantial Real Presense of Christ in the Eucharist is FALSE.
However, what
actually happens is the following:
Scenario 1: The NT Canon
A: Athanasius (and/or the Fathers) believed the New Testament canon consited of the 27 NT books.
B: I (and/or my church) believe the New Testament canon consists of the 27 NT books.
THEREFORE, Athanasius’ (and/or the Fathers’) determination of the 27 book NT canon is TRUE.
Scenario 2: The Eucharist
A: Athanasius (and/or the Fathers) believed in the actual, substantial Real Presense of Christ in the Eucharist.
B: I (and/or my church) do not believe in the actual, substantial Real Presense of Christ in the Eucharist.
THEREFORE, Athanasius’ (and/or the Fathers’) belief in the actual, substantial Real Presense of Christ in the Eucharist is FALSE.
As the above clearly shows, while the person
claims to appeal to Athanasius (and/or the Fathers) as the authority for his belief that the 27 book NT canon is correct, what he
actually appeals to as authority is his own (or his church’s) pre-conceived belief. He merely uses Athanasius (and/or the Fathers) to support that belief because they agree with him in this instance.
If he was really appealing to the Fathers as authority for the NT canon, he would have to agree with them on the Eucharist as well in order to be logically consistent. Something is either authoritative or it is not. It can’t be authoritative on one point but not authoritative on another unless there is an appeal to a higher authority to determine when it is authoritative and when it is not. In this instance, that higher authority is the person’s (or his church’s) own pre-conceived belief. It can’t be the Bible in this instance because the question is dealing with what constitutes the proper composition of the Bible itself.