The three "great" Abrahamic faiths trace their origin to Abraham's willingness to sacrifice his son to God. Jews and Christians believe the story told in Genesis 22, in which God tells Abraham to kill his son, Isaac, for him.
God ... said, Take now thy son, thine only son Isaac, whom thou lovest, and .. and offer him there for a burnt offering. (Genesis 22:1)(Actually God was wrong about the "only son" thing, because Abraham, at the time, had two sons, not one. Abraham had already abandoned his first son, Ishmael, and his mother, Hagar -- with God's consent and blessing. So I guess Isaac was Abraham's only son, at least as far as he and God were concerned.)
God didn't make Abraham do the evil deed, though, substituting a ram instead at the last minute. But it was Abraham's willingness to do it that impressed God so much. Abraham passed the test because he was was willing to kill his son for God.
For his willingness to do the greatest evil any father could ever do, God blessed Abraham and made him the father of all believers.
Because thou hast done this thing, and hast not withheld thy son, thine only son ... I will bless thee ... And in thy seed shall all the nations of the earth be blessed; because thou hast obeyed my voice. Genesis 22:16Muslims also honor Abraham for his willingness to slaughter his son, but they believe it was a different son. They believe it was Ishmael, not Isaac, that God told Abraham to kill.
Here's the story from the Quran:
When (his son) was old enough to walk with him, (Abraham) said: O my dear son, I have seen in a dream that I must sacrifice thee. So look, what thinkest thou ? He said: O my father! Do that which thou art commanded. Allah willing, thou shalt find me of the steadfast. Quran 37:102(Notice, though, that the Quran doesn't actually name the son, so maybe a Muslim could believe that Isaac was the intended victim.)
The important thing, though, is this: Abraham was willing to kill his own son for God, and God rewarded him for it.
And then, of course, there's Jesus.
God supposedly tested himself, just like he did Abraham, only this time he went through with it. He killed his own son for some god-awful reason. I guess it was to keep himself from torturing others for stuff they didn't do or didn't believe. Or something like that.
Believers call it Good Friday.