If I had to choose the greatest error of reasoning that leads people to false conclusions and accompanying troubles, it would be the set of fallacies that make the assumption that certain things were true when they actually need to be proven. Sometimes, this is deliberately done, when someone engages in sophistry to justify a position and make opponents look bad. But many times, this is done simply because an individual assumes that there is a link between two things that does not actually exist. In fact, an assumption is something that is accepted as true, but without proof.
I think it’s a shame that the West has forgotten the Christian and Classical writings she was once based on, because—despite the propaganda which is determined to make the ancient and medieval times seem primitive and ignorant—they knew many things about reality that we have forgotten. Take Aristotle. He wrote over 2366 years ago about how people make this error.
Now begging the question […] may be done by assuming what is in question at once; it is also possible to make a transition to  other things which would naturally be proved through the [65a] thesis proposed, and demonstrate it through them, e.g. if A should be proved through B, and B through C, though it was natural that C should be proved through A: for it turns out that those who reason thus are proving A by means of itself. This is what those persons do who suppose  that they are constructing parallel straight lines: for they fail to see that they are assuming facts which it is impossible to demonstrate unless the parallels exist. So it turns out that those who reason thus merely say a particular thing is, if it is: in this way everything will be self-evident. But that is impossible.
[Aristotle, “Analytica Priora,” 64.2.30–65.1.9 in The Works of Aristotle, ed. W. D. Ross, trans. A. J. Jenkinson, vol. 1 (Oxford: The Clarendon Press, 1928).]
In other words, people make an assumption that the existence of certain incidents prove the point they want to make, but those incidents are all interpreted by the assumption that belief is true. when the accuracy of the claim is being questioned in the first place. One common example of this error is the canard that the only reason that can explain opposition to “same sex marriage” is “homophobia” and the only reason for opposition to contraceptives is a “misogynistic” attitude from bishops—and if they weren’t celibate, then teaching would be different. If one were to actually seek out what the Church taught, they would see that her motives are not the motives people claim she holds them for.
But this argument fails to examine the basis of Church teaching, that the existence of every Catholic teaching that says that the sexual act is only validly practiced in the marriage between one man and one woman where the openness to life is not violated and the spouse is not reduced to an object of pleasure. The Catholic teaching on the substance of the marital act rejects contraception, fornication, adultery, homosexual acts, rape, etc., because she teaches “This is what sex is for, and practices which reject this view cannot be done.” One can choose to accept or reject the Catholic teaching, of course, but to reject such teaching on the grounds of “homophobia” or “misogyny” are false grounds—because we deny that we hold our teachings for these motives.
Another common attack against the Church is based on the belief that the Church holds a teaching because of the political leaning of the men who are bishops and the Pope. Thus political liberals attack the bishops of being the “Republican Party at prayer,” on the grounds that opposition to abortion and “same sex marriage” are “conservative” positions. At the same time, political conservatives attack the Pope and bishops for speaking out on social justice issues like immigration, economic justice and ecology. The Pope is called a liberal, a Marxist, a Modernist, or a naïf who is “falling for” liberal propaganda.
But the premise is what needs to be proven—that opposition to sexual immorality or abortion is “conservative” and social justice is “liberal.” If the Pope or bishops have a motive which has nothing to do with American political categories, then the accusation has no basis. Again, every case cited which “proves” the Church is politically motivated is actually assuming the claim which has to be proven.
What is overlooked is the fact that the Church operates from the motive that she believes herself called to preach the Gospel to all nations, baptizing and teaching people to keep God’s commandments. She has been teaching these things long before Republicans, Democrats or Marxists ever existed. Try reading St. John Chrysostom on the obligation to the poor, for example. He lived over 1500 years ago and taught about the Church teaching in such a way which would be unpopular to some Americans today. Likewise, the Church Fathers who spoke about sexual immorality and abortion that was as widespread as the practices today. The motive was not to persuade members of the Roman Empire to “vote Republican.” It was to persuade individuals to repent and believe in the Gospel.
Ultimately, the attacks against the teaching of the Church tend towards making assumptions about the motives and intentions about the people who implement them. The individual who opposes these teachings alleges bad will, but never considers the possibility that there could be another option. Sure, we can get a member of the Church who does dissent and claims to teach “truth” in opposition to the magisterium, but it is wrong to assume that unfamiliar behavior is willful disobedience. We need to consider other options—of misunderstanding on our part, of a lack of information, of a mistake in judgment etc.
If we refuse to consider whether the assumptions we make are true, we run the risk of going very far astray from our faith, making a shipwreck while considering ourselves to be sailing smoothly.