The forced conversions perpetrated against the Jews of the Iberian Peninsula did not occur in a vacuum. The theological foundations in medieval Christianity had been laid centuries before. The presence of Jews in Christian lands created a theological for many Christian theologians.
The solution to the challenge of having infidels in their midst created two complimentary approaches. The first was conversion. If that failed, the second option was expulsion.
In the 10th century the choice between expulsion and conversion was addressed by Pope Leo VII who declared that “You should never cease preaching to them, with all the sagacity, prudent counsel and reverence toward G-d, the belief in the Sacred Trinity, and the mystery of the Lord’s incarnation. If they wholeheartedly wish to believe and become baptized, we shall offer thanks and high praise to the omnipotent G-d. Should they, however, refuse to believe, you shall expel them from your states with our permission. For we ought not to associate with the L-rd’s enemies, as the Apostle says: ‘Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness…or what part hath the that believeth with an infidel?’ [II Cor. 6:14-15]. But you must not baptize with force, and without their wish and request. For it is written: ‘Give not that which is holy unto the dogs, neither cast ye your pearls before swine, lest they trample them under their feet.” 
As long as Jews were in critical positions in the struggle between Islamic and Christian kingdoms, such theological concerns were often set aside for pragmatic reasons. As soon as Christian hegemony was secure, the Jewish question arose again.
By Rabbi Juan Bejarano-Gutierrez the director of the B’nei Anusim Center for Education and author of What is Kosher?
 Salo Wittmayer Baron, A Social and the Religious History of the Jews: Volume IV (Philadelphia: Jewish Publication Society, 1957), p.6.