Reb Eliyahu di Vidas, student of the Ramak and the AriZal, writes in his classic work Reishis Chochma, that a Fear of HaShem can be acquired only by understanding who He is. A servant who doesn't know his master will not serve him properly.
(ראשית חכמה שער היראה פ"א)
Reb Avraham Ibn Ezra was traveling incognito and a certain Yid invited him in. He gave his guest a nice room and fine food, and treated him well.
Somehow word got out that the visitor was none other than the famous Ibn Ezra, so all the scholars of the town lined up outside the house to meet the great gaon and bask in his teachings. When the host found out who his guest was, he fell before Ibn Ezra and asked forgiveness for not having treated him properly.
"Why are you begging forgiveness?" asked the gaon. "You treated me wonderfully!"
"Yes," said the host, "but had I known who you were, I would have treated you even better."
Hearing this, the Ibn Ezra raised his eyes heavenward and said, "Ribbono shel Olam, I too must ask forgiveness for not having served You properly. Had I known Your true greatness, I would have served you much better."
(טללי תשובה ע' 533)
This, the Rebbe Rashab explains, was one of the reasons for the revelation of Kabbala and Chassidus in recent generations. Yidden of earlier times had lofty neshamos and they could bring themselves to love and fear HaShem, unaided. With the spiritual devaluation of successive generations, the potent spiritual revelation of the Zohar was revealed, to make people aware of HaShem. Later, due to a further descent, the Baal Shem Tov and the Alter Rebbe were sent to bring introduce people to a deeper recognition and understanding and of HaShem.
To use a parable: A person who is healthy needs only regular food and drink to thrive; a person whose health is impaired also needs medicines. Similarly, in earlier generations, when klal Yisroel was spiritually healthy, it thrived on the simple meaning of the Torah. The secrets of the Torah were reserved for the spiritual elite. However, as the spiritual health of our nation gradually deteriorated, a strong medicine was needed.
(קונ' עה"ח פי"ג, לקו"ש ח"ל ע' 170, אג"ק ח"ד ע' שע"ז)
A Yid living in the times of the Alter Rebbe could boost his neshama to withstand the challenges of golus with concise and potent chassidic teachings, such as those of the Baal Shem Tov and the Maggid. The thorough understanding of Chassidus, as introduced by Alter Rebbe in Chabad Chassidus, was then a luxury, a foretaste of the revelations of Moshiach.
However, as the darkness of golus has deepened, the only way to withstand and survive its increasing challenges is to study and comprehend G-dliness.
(לקו"ש ח"ל ע' 170)
via Lma’an Yishme’u