It is a common demand to prove that the Church has always taught X, where X is some doctrine that Protestants disagree with (they never demand such proof for things they agree with :))
Of course in matters of faith, and even in matters of ancient history, we cannot offer proof, only evidence. So here is the generic line of evidence defending the claim that the Church has always taught X:
There is ample evidence that the Magisterium of the Church has fought heresies when they have appeared, both through Councils and through Papal writings. I don’t think anybody would disagree with this.
Heresies begin as local errors and spread. They do not come into existence full-blown and universal. Thus there is a time after a heresy begins when there are people who do not believe the heresy.
At some point, it becomes a matter historical fact that doctrine X is taught in the Church.
This just leaves the question, was X taught before that point in #3?
To claim YES (as Catholics do) without direct evidence (for we are before that point specified in #3), we rely on the facts #1 and #2, that the Church did and does fight heresy, and that no heresy can spread instantaneously. Therefore there must be a time in the life of every heresy when some portion of the Magisterium recognizes the heresy as a heresy. Thus the historical fact that the Church did not at that point attack the idea X is evidence that X was not considered a heresy by the Magisterium at the point it appeared in the historical record, and by implication, neither was it considered heresy at any previous point.
By contrast, to claim NO in the face of #1, #2 and #3, one must believe that the entire Church, which was at that time not teaching heresy, became suddenly incapable of recognizing heresy. It is to claim that, at the same time the Church was fighting some heresies, it began turing a blind eye to others. Such an abrupt and universal change behavior would be inexplicable.
Note carefully that in one sense both of these arguments, for YES and for NO, must be arguments from silence. Therefore it is no good to claim the other argument is argument from silence. That charge cuts both ways.
So, the question comes down to what is more reasonable to believe: That the Church did not attack “new” (in the historical record) idea X because it was already believed when it first entered the historical record, or that the (error-free until that moment) Church did not attack “new heresy” X because it was suddenly and totally incapable of recognizing a new heresy in its midst?
The former seems vastly more defensible in explaining the facts at hand than does the latter.