I saw wise men, men of understanding and piety, engaging in long discourse, who have written great and terrible things in their books and epistles. And once something is written it cannot be concealed any more, for often it will get lost or the author will die and the letters will pass into the hands of scoffers and idiots, and the name of God is profaned.
[Isaac the Blind to Nahmanides and Jonah Gerondi, quoted in Harvey J. Hames, The Art of Conversion: Christianity and Kabbalah in the Thirteenth Century. Brill (Boston: 2000) p. 51.]
So seriously did the great Kabbalist take this issue that he refused to write down anything about his views in a letter even to a Halakhic authority as great as Nahmanides, when Nahmanides had asked him to clarify a point. Instead, he sent one of his disciples to clarify in person. True Kabbalah, for Isaac, is not something that can be learned from books; it is passed as a living tradition from ear to ear and in no other way. He would be horrified at the popularizations of Kabbalah we have to deal with today.
However, this was as serious an issue in the thirteenth century as it is today, and perhaps more so, because the very future of Judaism was at stake. The source of trouble -- it is the source of most of the intellectual and religious troubles in the thirteenth century -- was the flood of Aristotle (and Aristotelian commentators) being injected into Christian Europe at the time. The Christians were not the only ones who had to deal with it. After all, one of the great figures who became prevalent in Europe at this time (through his written works) was Rabbi Moses himself, the great Maimonides, who had used Aristotelian philosophy to clarify theological matters. This led to the introduction of a trend in certain parts of Jewry to 'philosophization' of Jewish thought and life. These people are often called the 'Maimonidean' party, and the controversy they sparked is often called the 'Maimonidean' controversy. The label is very misleading; any comparison of Maimonides with (say) the very Aristotelian Gersonides will show very clearly that Maimonides was not so clearly in the philosophizing camp. Further, the big worry about the 'Maimonideans' was that they were replacing Jewish tradition with Greek tradition, and Nahmanides, a great and well-respected opponent of the philosophizers, recognized that Maimonides was actually an ally against the philosophizers, rather than a philosophizer himself. Nahmanides was surely right; but his was a minority view at the time, and Maimonides himself came up for considerable criticism.
A battle, even an intellectual and cultural battle, needs opposition. Who was opposing the philosophizing camp? The Kabbalists; which is not surprising, since the sort of Jewish reform they were advocating was in some ways diametrically opposed to the sort of reform advocated by the philosophizers. The battle wasn't, properly speaking, between innovators and traditionalists -- it was pretty clear on both sides that both groups were innovating, because they were both advocating reform, and with reform there necessarily comes innovation to deal with new situations. The real heart of the dispute was over which side was the genuine bearer of the living tradition. Who was carrying forward the torch of authentic Judaism? Who was the divergent branch? Which innovations developed the tradition rather than deviating from it? The Kabbalists' innovation was the use of Kabbalah not merely as an esoteric tradition among rabbis, but as a way of adding additional cohesion and richness to Jewish thought and life. The philosophizer's innovation was the use of Greek reasoning in a similar way. Both claimed, as a rule, to be presenting the truly Jewish path.
The controversy came very near to tearing Judaism apart. Nahmanides lamented at one point that instead of having one Torah, they had split it into two Torahs. While he was very definitely in the Kabbalist camp, he nonetheless firmly insisted that both groups treat each other respect (he vehmently defended Maimonides on a number of occasions, despite his many disagreements with him), and held that there was room, under Halakhah, for both of them. In part because Jews on both sides took Halakhah seriously, and in part because Nahmanides was the foremost Halakhic scholar of his times, it was Nahmanides's approach that (generally) won out; because of him, Jewish thought made room for both Kabbalah and philosophy as important contributions to Jewish life.
But in the meantime, people like Isaac the Blind, who were definitely in the Kabbalist camp, but very much of the old school, were profoundly worried at the changes afoot. Much of the infusion of Kabbalah into the broader Jewish consciousness is due to the dispute between the Kabbalists and the philosophizers. In order to attack, refute, and respond to the philosophizing camp, the Kabbalists were forced to make the esoteric doctrines of Kabbalah more and more exoteric. Hence Isaac's worry about the men of understanding writing "great and terrible things" -- things that should be reserved for oral tradition, and not committed to writing.
The paradox of Kabbalah as we know it dates from that time. For on the one hand, it has spread out and out, so that Kabbalistic ideas have become more and more common (and not just among Jews). On the other, Kabbalah is esoteric; it is discourse on mysteries, a discourse that cannot be carried on a written page, but only in the careful and wise guidance of someone who knows them at first hand. A Kabbalah that is common knowledge is not the true Kabbalah.