“Now, O Israel, what does Hashem, your G-d, ask of you? Only to fear Hashem, your G-d, to go in all His ways and to love Him, and to serve Hashem your G-d, with all your heart and with all your soul” (Devarim 10:12)
The Ohr Hachaim says that fear is the opening through which love can enter.
Rabbi Chanina says: Everything is in the hands of Heaven except for the fear of Heaven, as the pasuk states, “What does Hashem… ask of you? Only to fear Hashem” (Berachot 33b).
“And he said, ‘Do not stretch out your hand against the youth nor do anything to him for now I know that you are a G-d-fearing man, since you have not withheld your son, your only one, from Me” (Bereishit 22:12)
Rashi expounds on the words For now I know: “Now I have something with which to answer the Satan and the nations, who wonder what the cause of My love for you is. I have a reason now that they see that you fear Hashem.”
The Recanti (Bereishit 22:12) asks the following. We know that Avraham was called Hashem’s beloved one. Why wasn’t he praised for his outstanding quality, that of loving-kindness, rather than for his fear of Heaven? We know that all of Avraham’s actions were motivated by his tremendous love for Hashem. The difference between loving and fearing is as vast as the difference between light and dark.
The Masters of Kabbalah teach that there are two types of fear of Heaven. There is inner fear and there is external fear. External fear of Heaven is inferior to love of Heaven, whereas internal fear of Heaven is superior to love of Heaven. How is this? External fear is fear of sin. One is afraid to sin lest he be punished by the King. Inner fear, however, indicates a person’s perception of Hashem’s greatness and His vast attributes. When a person reaches this level, he feels inadequate. He is overcome with fear, for perhaps he is not worthy of standing in His Presence. Hashem’s awesomeness and his own smallness keep him constantly fearful lest he transgress His will and be severely punished. He is thus constantly worried that the King will find a deficiency in him. Out of his extreme love for his King, he is always afraid of being found unworthy.
This type of fear is superior to love. Avraham acquired this fear, as the pasuk states, “You are a G-d-fearing man.” One who has reached this level of fear of Hashem considers his wife, his children, and, indeed, his very self, as nothing at all to sacrifice for Hashem’s sake. He regards such a thing as someone who buys a thousand blocks of gold for one cent.
The Recanti (ibid.) continues: This inner fear of Hashem is mentioned in the following pesukim, among others: “The beginning of wisdom is fear of Hashem” (Mishlei 9:10); “Praiseworthy is the man who fears Hashem” (Tehillim 112:1); “Praiseworthy is each person who fears Hashem” (ibid. 128:1); “Hashem, your G-d, shall you fear” (Devarim 10:20); “You shall revere your G-d” (Vayikra 19:32); “What does Hashem, your G-d, ask of you? Only to fear Hashem” (Devarim 10:12).
The Ohr Yechezkel states that learning Torah without possessing fear of Heaven is merely like any other intellectual exercise. But learning Torah when one does possess fear of Heaven is the most cherished and superior form of study.
The Lev Eliyahu (III, 17) writes that without fear of punishment, it is impossible for one to vanquish his Yetzer Hara and correct his character flaws, such as passion, anger… One should literally be afraid to sin, accepting upon himself fear of Heaven, understanding that if he transgresses, he will pay dearly. For this reason, Hashem held Har Sinai over the heads of Bnei Yisrael like a barrel, compelling them to accept the Torah.