If Ktav Ashurit was not introduced until Ezra, what Ktav was the Torah given in? Also, how can mystical meaning be derived from the Hebrew letters, as is currently done by many Kabbalists, if the shape of the Hebrew letters was different from what we use today?
Shalom, Thank you for your question. You are correct that there is a major opinion that ktav ashurit was introduced by the prophet Ezra and until then the Jewish people used ktav ivri (see the Talmud Sanhedrin 21b-22a). However, the matter is not so simple! The Talmud brings three different opinions about this question – 1. That Ezra introduced the change in script from ivri to ashurit, as above. (R. Yosi and Mar Zutrah) 2. That the Torah was originally given in ashurit, which was used until the end of the first temple, then forgotten until Ezra re-instated it. (Rebbi) 3. That there was no change in the script of the Torah, and that we always used ktav assurit. (R. Shimon ben Elazar – also held by Rav, Shmuel, R. Yochanan, and R. Ashi) The Geonim in their response (358) held the third view. The Rambam also holds this view (see commentary to Mishna Yadaim 2,5). According to this, the questions you raise do not apply – the Torah was always in the same letters we have today – Assurit. However, even according to the first opinion there are at least two other explanations that may help us. Firstly, some say that the Torah was given in Assurit, but Moshe and the Jewish people did not want to use such a holy script for everyday use. As such, they used ktav Ivri – until the time of Ezra, when it became acceptable to use ktav ashurit at all times. According to this, even the first opinion agrees that the inherent holiness of the letters applies to Asssurit – and when we say that Ezra "changed" the text to assurit from ivri, we only mean for general open use. Another explanation is found in the Radbaz (III, 882) where he explains that the first tablets were in ktav ashuri, but after the sin of the golden calf, the second tablets were in ktav ivri, which was used until the time of Ezra. This explains the holiness of the letters of ktav ashurit, together with the opinion that Ezra made the change from ivri to ashuri. You may be interested to read about this (and the general holiness of the letters) in a wonderful book called "The Wisdom in the Hebrew Alphabet" by Rabbi M.L. Munk (published by Artscroll) [on which I based this answer to you]. Also, Rav Kook zt"l has a very interesting understanding of this issue that can be found (in Hebrew) in his introduction to his Eiyn Aiyah. Blessings.