Antisemitism, as defined by radically different treatment compared to their Christain counterparts
Was the antisemitism based on the specific opinions of thought leaders
The greatest doctor of the Catholic Church, St. Thomas Aquinas (✝1274), when discussing the sin of unbelief, asks "Whether the children of Jews and other unbelievers ought to be baptized against their parents' will?" (Summa Theologica II-II q. 10 a. 12 co.). He says they should not. He gives one argument from authority:
The custom of the Church has very great authority and ought to be jealously observed in all things, since the very doctrine of catholic doctors derives its authority from the Church. Hence we ought to abide by the authority of the Church rather than by that of an Augustine or a Jerome or of any doctor whatever. Now it was never the custom of the Church to baptize the children of the Jews against the will of their parents, although at times past there have been many very powerful catholic princes like Constantine and Theodosius, with whom most holy bishops have been on most friendly terms, as Sylvester with Constantine, and Ambrose with Theodosius, who would certainly not have failed to obtain this favor from them if it had been at all reasonable. It seems therefore hazardous to repeat this assertion, that the children of Jews should be baptized against their parents' wishes, in contradiction to the Church's custom observed hitherto.
and two arguments from reason:
on account of the danger to the faith. For children baptized before coming to the use of reason, afterwards when they come to perfect age, might easily be persuaded by their parents to renounce what they had unknowingly embraced; and this would be detrimental to the faith.
it is against natural justice.
Also, St. Thomas Aquinas didn't consider Jewish worship to be idolatrous, as he did for other unbelievers (Summa Theologica II-II q. 10 a. 10 "Whether the rites of unbelievers ought to be tolerated?" co.):
from the fact that the Jews observe their rites, which, of old, foreshadowed the truth of the faith which we hold, there follows this good—that our very enemies bear witness to our faith, and that our faith is represented in a figure, so to speak. For this reason they are tolerated in the observance of their rites.
On the other hand, the rites of other unbelievers, which are neither truthful nor profitable are by no means to be tolerated, except perchance in order to avoid an evil, e.g. the scandal or disturbance that might ensue, or some hindrance to the salvation of those who if they were unmolested might gradually be converted to the faith. For this reason the Church, at times, has tolerated the rites even of heretics and pagans, when unbelievers were very numerous.
cf. Thomas Aquinas on the Jews by Boguslawski, O.P.
or was it derived from the actual teachings of Christianity?
St. Thomas's letter to Margaret II, Countess of Flanders, who asked him about usury and the Jews, answers her question about "Whether it is good that Jews throughout her province be compelled to wear a sign distinguishing them from Christians." He writes:
according to a statute of the general Council, Jews of each sex in all Christian provinces, and all the time, should be distinguished from other people by some clothing. This is also mandated to them by their own law, namely that they make for themselves fringes on the four corners of their cloaks, through which they are distinguished from others.
The blame of collective guilt from the killing of Jesus Christ placed on the Jews
St. Thomas distinguishes between the Jewish rulers (who knew who Christ was) and the common people (who didn't know). Discussing the gravity of the sin of crucifying Christ, he writes (Summa Theologica III q. 47 a. 6 co.):
the rulers of the Jews knew that He was the Christ: and if there was any ignorance in them, it was affected ignorance, which could not excuse them. Therefore their sin was the most grievous, both on account of the kind of sin, as well as from the malice of their will. The Jews also of the common order sinned most grievously as to the kind of their sin: yet in one respect their crime was lessened by reason of their ignorance. Hence Bede, commenting on Lk. 23:34, "Father, forgive them, for they know not what they do," says: "He prays for them who know not what they are doing, as having the zeal of God, but not according to knowledge." But the sin of the Gentiles, by whose hands He was crucified, was much more excusable, since they had no knowledge of the Law.
Contemporary Catholic historian E. Michael Jones defines sicut Iudeis non in his Jewish Revolutionary Spirit introduction, p. 21:
The Catholic response to the revolutionary Jewish rejection of Logos came to be known as "Sicut Iudeis non…," a doctrine codified by Pope Gregory the Great and reiterated by virtually every pope after him. According to "Sicut Iudeis non…," no one has the right to harm Jews or disrupt their worship services, but the Jews have, likewise, no right to corrupt the faith or morals of Christians or subvert Christian societies.
Jones's Goy Guide to World History video series also discusses sicut Iudeis non.
For more info and resources, see this Christianity StackExchange answer
and William Thomas Walsh
's history books, such as The Last Crusader: Isabella of Spain (1451-1504)
and Characters of the Inquisition