Rav Chaim Vital on the importance of learning the inner parts of Torah [Chassidus and Kabbalah]:
Why is the exile taking so long to end? Why has Moshiach not yet come? I have found the following explanation: It is due to the lack of study of the inner parts of Torah. One who places his focus to only study the revealed aspects of the Torah, the Mishneh and Talmud Bavli, its physical laws and details, and does not give time to study also the inner parts of the Torah, is considered to be studying for the sake of reward. He is like a body which sits in darkness without the light of Hashem. This was precisely the sin of Adam Harishon who chose to eat from the tree of good and evil. This means that he chose to spend his time only studying the revealed aspects of Torah and repulsed from taking from the tree of life, which is the study of Kaballa. This was also the sin of the Eiruv Rav, who asked Moshe to only teach them the revealed aspects of the Torah and not its secrets, as they feared it would shorten their lives. This is also the mistaken belief of some people today that studying the inner aspects of the Torah can lead to death prior to one’s time. This is untrue. On the contrary, the focus in learning only the revealed aspects of Torah, and ignoring the inner aspects of Torah, is what caused the destruction of the first and second Temple, and is the cause of our long and bitter exile. Only through tasting the tree of life, which is the study of Kablla, will the Jewish people leave the exile. The sin of not learning the inner aspects of Torah began with Adam Harishon and when we do Teshuvah to study this wisdom with love we will be redeemed. Those who despise learning the inner aspects of Torah do not receive Divine success in their learning of the revealed aspects of Torah and often end up permitting that which is forbidden and prohibiting that which is permitted.
 Rav Chaim Vital in Shaar Hahakdamos, printed in the Hosafos of Kunrtus Eitz Chaim