Abner of Burgos on Karaites: It was not long ago that these Jews of the Kingdom of Castile and most of the Jews of Spain were Sadducees and heretics, as the sage Abraham ibn Ezra wrote in his commentary on the Torah.
So wrote Moses of Leon in the letter he composed in order to contradict the Sadducees, in which he said that it was the year 4938 of creation (1178 CE) in which they converted to believe in the faith they now have in the Talmud Bavli, not by their own free will, but rather because of the uproar that there was between Sadducees and Pharisees in the town of Carrión (presumably Carrión de los Condes), but the Sadducees were more powerful and they did not light candles on the night before the Shabbat through all the town.
There were among the Pharisees three honoured men who always complained that the Law was not observed among the people, and (they complained) so much that one of them became zealous for God and lit candles publicly for the Shabbat.
And that was not the custom of the Sadducees…and there was a great outcry in the Jewish neighbourhood, because people thought that he violated the Shabbat.
And the news of this arrived to the head rabbi of the Sadducees, who lived in Burgos, and he ordered that (the one who lit the candle) be arrested.
So, the Sadducees and Pharisees had to go to the king Don Alfonso VIII.
Among the Pharisees there was one Jew, physician of the king, named Joseph ibn Alfacar, from Granada.
And the king did what he wanted, and so the king ordered all the Sadducees to turn to the law of the Pharisees, and so they all turned, against their will, to this Pharisee law, which they now follow.
By such accidents as these the Jews in each age turn from one belief to another and they wander in exile like sheep without a shepherd, ever since they left the true shepherd…

Ibn Daud on Karaites: He asked the king to forbid the heretics to open their mouths throughout the land of Castile, and the king commanded that this be done.
Accordingly, the heretics were suppressed and have not been able to raise their heads any longer.
Indeed, they are dwindling steadily.

Yosef ben Todros on Karaites: He removed from our provinces the abominations of the heretics until he succeeded in destroying their fortresses and pulled down to earth their glory and there was not on the day of God’s wrath a remnant when he executed judgment upon them and upon their books.

Abner on Talmud burning: There are found among the Jews many man who deny their Talmud…they demand of the bishops and other Christian princes that they burn that Talmud…such men flee and distance themselves from disputing with a Christian, because they believe that he will prove to them (these things) from their Talmud.
And this is so that what they deny about their Talmud would not be known, and they strengthen themselves in their heresy when they see the disagreements that the Jews have in their faith…This is because it is amazing to find ten men among twenty of them who agree on all issues of their faith.  

Abner on Pharisees and Sadducees: Some of the Jews are those who believe in all of the Babylonian Talmud and they are called Pharisees, and some others are those who do not believe in all of the Babylonian Talmud, but only part of it, and others are those who do not believe anything of the Talmud, but only (believe in) the Bible, and they are called Kutim (Cutheans), and there are others who are called Sadducees who do not believe that the souls of men remain after death, and there are others of them who believe that the soul remains in some way, but they do not believe that there will be reward or punishment….  

Abner on forced conversions: It is necessary to proceed against (the Jew) gently and to move him from (the wrong) reasoning little by little, as is appropriate for human nature.
It is necessary to convince him.

This is because of the damnation and ongoing captivity that you were sentenced with, in which you should have tribulations with gentleness and without a great harshness.
For if you had it with great harshness in such a way that you could not endure or that you were wiped out with one blow, in this way your dishonour, which was to be ongoing, would be ended, and the honour of God which now exists and which gives you trials in every age would be limited.

God takes away that hard switch that could give you wisdom and turn you to good, and this is always because it is not the will of God to give you salvation or remove from you the universal and eternal dishonour that he gave you as a sentence.

If the Jews had tribulations that were bad enough or strong enough so that they had to convert to the law of the Christians or the Moors, or such that they were wiped out from the world, those evils would devolve by nature onto the Christians and Moors, and these would lose the benefit and divine protection that they have and account of believing in the Christ…moreover, it is necessary that God keeps the Jews in the world, like he keeps the devils, so that they can be arms of his wrath and messengers of his anger in order to make bad Christians and Moors (who deserve it) trip and fall into punishment and to send them temptations and doubts in their faith, hoping to wipe them out from the world.
See now how great and marvellous are the works of God, who chose the people of Israel from among the other nations, for his service both in substance and in accident.
For in them was the salvation of the world in substance, in the sense that (God) made the Christ be born among them, along with the other righteous ones who were born and are born in all ages in order to confirm their faith and their salvation.
And also in that was salvation in accident, in the sense that (God) keeps the bad ones among them in order to be for a curse and a judgment in order to castigate and give punishment through them to the sinners that he wants to punish and make turn through penance and to whom give salvation in the next world. 

Abner of Burgos - Jewish convert to Christianity, after his conversion was known as Alfonso de Valladolid (1270 – 1346)