What does "Fear of God" mean?

Nechama Leibowitz makes an amazing observation: Every time someone is described as "God-fearing" or not "God-fearing" in the Torah, it refers to how they treat the weak or defenseless. For example, Genesis 42:18 in reference to Joseph being kind to the brothers, or Exodus 1:17 regarding to the midwives saving the babies, or Deuteronomy 25:17-18, in reference to Amalek attacking the weak of the Jewish people.

Meaning, the definition of "fearing God" in the Biblical sense is directly correlated with how you treat people who need your help.

Incidentally, she "paskens" that the war against Amalek is no longer an ethnic or racial one, but against those who do not fear God in this sense. She quotes Rashi on Exodus 17:16 which becomes so much more powerful when read that God's throne is incomplete while "Amalek" (cruelty to the defenseless) still lives.

However, as some of my friends point out, there are many times that the phrase "fear of God" does not seem to refer to the protection of the weak. See, for example, the angel stopping Abraham from slaughtering Isaac, declaring that now he can see that he fears God, Genesis 22:12

"Do not lay a hand on the boy," he said. "Do not do anything to him. Now I know that you fear God, because you have not withheld from me your son, your only son."

 But perhaps Professor Schrader is correct in this very interesting article, and the true test of the sacrifice of Isaac was how he withheld the slaughtering of his son, despite the social acceptance of it, by the word of God. So the "fear of God" was that he protected his defenseless son.

Alternatively, I'd say that now God "knows" he was willing to sacrifice the son he loved and allow his older and less loved son to be his heir to the nation, a function of his fear of God to care about the weak. But that is a big chiddush.

A friend asked about Deuteronomy 10:12-13, "And now, Israel, what does the Lord your God require of you, but to fear the Lord your God, to walk in all His ways and to love Him, to serve the Lord your God with all your heart and with all your soul, and to keep the commandments of the Lord and His statutes which I command you today for your good?"

This doesn't preclude the above definition, in fact in 10:18, Moshe continues by describing God as the helper of the needy and the defenseless, and caps it off in 10:20 by repeating the need to fear the Lord.

It should be noted that Nechama Liebowitz was very careful with her words and said this interpretation only on regard to descriptions of people or groups as God-fearing, so these verses don't have to do with that at all.