I have the following sort of example in mind. Suppose there’s a middle aged person, who grew up in an atheist household. Suppose he’s, naturally, been living in typical materialist fashion - that is, in sin and error like a fish in water. The internal forum is impossible to know with certainty, but at some point from outward appearances it looks like he’s come across the faith, and, after educating himself and ostensibly reforming his life in preparation, has decided he wishes to join the Catholic Church.

Because of the iniquity of the age, and in particular because of the prevalence of such iniquity’s chief means of infestation being by conspiracy (i.e. by malice, or intentional evil, which is the worst kind in contradistinction to that evil done by ignorance or done by passion), the parish priest contacted by the man denies him acceptance to the Church. Prudentially, the priest judges that conditions are such that he cannot risk the souls of his flock, whom he knows to at least be of general good will, by introducing someone who may not be. Or taking an even stronger stance, the priest does believe the man is of general good will, but because of the man’s history, the priest judges such latent particularities of his special kind of former iniquity mean that if the man falls into iniquity again, he is liable to fall extremely far and take many of the flock with him, and so the priest refuses to admit the man to the Catholic Church.

Is such action by the priest within his proper authority, and by extension, within the authority of the Church?

I ask because I recall once hearing a priest in a sermon in passing refer to the Church admitting converts as an act of charity. I think I can see how this baseline attitude toward hopeful converts makes logical sense; was this the historical baseline attitude?

Sounds right, it kind of begins to sound like looking for reasons to exclude people for the sake of excluding them, to say “allowing converts into the Church is an act of charity”, if just left at that.

Perhaps it’s better, to not mislead, to refrain from saying such in isolation, but always qualify it with “and charity, being the highest theological virtue, is obligatory” or something. That way one avoids identification with the attitude of Judaism.

As for avoiding identification with the Eastern attitude, like you point out, they do that lifting on their own - ironically to their own cultural losses, not participating in the time and place of highest culture ever created, the unified Christendom of Western Europe 1200-1400. Forever indulging in ethnic pride, they threw away their participation in the greatest cultural pride in man’s history, and so not even on earth do they win anything.

It falls to the jurisdiction of the Church, and her pastors, to do a certain kind of vetting of converts: mainly, pastors must ensure that those who wish to convert do in fact wish to convert.  However, the bar for this is very low.  As long as someone says they want to convert and doesn't give any significant indication otherwise, that is all a priest needs to know.  

It is possible for a priest to fail in making this judgment, of course. In the case of a priest who erroneously refuses to admit someone to the Church (either refusing to receive someone already baptized, or refusing even to baptize), the refused man could either go to another priest or, if this is not possible, he might qualify for being a member in voto (in desire). This sort of relationship to the Church falls short of actual membership and such a person would not be "owed" the sacraments or any of the other treasures of the Church.  Justified catechumens and those who are unjustly excommunicated are members in voto. Hopefully that helps.

It falls to the jurisdiction of the Church, and her pastors, to do a certain kind of vetting of converts: mainly, pastors must ensure that those who wish to convert do in fact wish to convert.  However, the bar for this is very low.  As long as someone says they want to convert and doesn't give any significant indication otherwise, that is all a priest needs to know. 

It is possible for a priest to fail in making this judgment, of course. In the case of a priest who erroneously refuses to admit someone to the Church (either refusing to receive someone already baptized, or refusing even to baptize), the refused man could either go to another priest or, if this is not possible, he might qualify for being a member in voto (in desire). This sort of relationship to the Church falls short of actual membership and such a person would not be "owed" the sacraments or any of the other treasures of the Church.  Justified catechumens and those who are unjustly excommunicated are members in voto. Hopefully that helps.

Ok thanks.

So far as I could find there were times and places of wariness toward converts - namely, in Spain for obvious reasons given that country’s history, and in crusader states likewise. But as for wariness regarding admission, it seems that was never a rule or norm - only wariness for apostasy afterward, which is not a matter unique to converts, and so not really a matter worthy of any special consideration at all.

Excessively weeding out potential converts based on potential actions far in the future, or intentions one cannot possibly discover - as is the nature of human intent generally, other than one’s own - would seem to fall under excess solicitude according to St. Thomas, which is actually opposed to prudence:

“For every time has its own fitting proper solicitude; thus solicitude about the crops belongs to the summer time, and solicitude about the vintage to the time of autumn. Accordingly if a man were solicitous about the vintage during the summer, he would be needlessly forestalling the solicitude belonging to a future time. Hence Our Lord forbids such like excessive solicitude, saying: "Be . . . not solicitous for tomorrow," wherefore He adds, "for the morrow will be solicitous for itself," that is to say, the morrow will have its own solicitude, which will be burden enough for the soul.”