The slogan "saved by faith, not be works" does not describe the reality. It was never intended to. It was intended as a shorthand to contrast an opposing reality of "saved by works". Let's start with an easy comparison. I do not think that Muslims would call this an unfair description of their faith, but I read that 'Allah' weighs your heart on a scale against a feather with your good deeds making it lighter and your evil deeds making it heavier. If your heart is heavier than the feather, you go someplace bad and if your heart is lighter, then you go someplace good. This is why it is important for a Muslim to follow the 5 pillars of Islam. They are acts of goodness required to be right with 'Allah'. This is a salvation by works. If you cannot do the works, then you cannot be saved and your good works must outweigh your bad works.
Protestant Christianity, in contrast, points to the Bible and claims that the standard is not good, God demands perfect. One strike and you go to hell. It then claims that even your best 'good deeds' fall so short of perfect that they are like 'filthy rags' to God (FYI: filthy rags is a polite translation of menstrual cloth ... your good works are like a used tampon to God ... Yuck). So Christianity claims that your sins are not forgiven because of the good things that you do, but because of grace (an unearned gift; a present) that God gives you because you have faith in Jesus.
Here it is explained in Ephesians Chapter 2:
Ephesians 2:1 As for you, you were dead in your transgressions and sins, 2 in which you used to live when you followed the ways of this world and of the ruler of the kingdom of the air, the spirit who is now at work in those who are disobedient. 3 All of us also lived among them at one time, gratifying the cravings of our flesh[fn] and following its desires and thoughts. Like the rest, we were by nature deserving of wrath. 4 But because of his great love for us, God, who is rich in mercy, 5 made us alive with Christ even when we were dead in transgressions—it is by grace you have been saved. 6 And God raised us up with Christ and seated us with him in the heavenly realms in Christ Jesus, 7 in order that in the coming ages he might show the incomparable riches of his grace, expressed in his kindness to us in Christ Jesus. 8 For it is by grace you have been saved, through faith—and this is not from yourselves, it is the gift of God— 9 not by works, so that no one can boast. 10 For we are God’s handiwork, created in Christ Jesus to do good works, which God prepared in advance for us to do.
Notice in verse 8, you are saved by grace. In verse 9 not be works, because God does not want people bragging about what THEY have done to become such wonderful people. In verse 10 then, after you are saved by grace, you have good works that God prepared for you to do.
Christianity teaches that we do not do good works in order that God will love us; it teaches that God already loves us and we are therefore motivated by gratitude to want to do good things. So the good works are the fruit that grows from the tree, not the roots that give the tree life. God's love is the root that allows us to live and grow and produce the fruit of good works.
I sure do:
Genesis 3:13 Then the LORD God said to the woman, “What is this you have done?”
The woman said, “The serpent deceived me, and I ate.”
14 So the LORD God said to the serpent, “Because you have done this,
“Cursed are you above all livestock and all wild animals!
You will crawl on your belly and you will eat dust all the days of your life.
15 And I will put enmity between you and the woman, and between your offspring and hers; he will crush your head, and you will strike his heel.”
Notice carefully several things in the curse on the talking serpent (which the New Testement clearly states is the Devil). In v.15 there is an interesting feature of the hatred between the Devil and the "offspring" (literally 'seed') in that the word is singular, not plural. There is a specific promise (prophecy) given by God right at the fall that HE (a singular male someone) descended from THE WOMAN will be born and 'crush the head' of the serpent (Devil), but the serpent will wound him. The details are not revealed until later, and Genesis was not written until the time of Moses, but if true, it represents a history that may have been oral in the time of Abraham, who spoke with the LORD directly.
So Abraham might not have known all of the details, but he knew that God had promised that a Son of Eve would be born and break the curse (crush the head of the serpent) while being wounded in the process. Christians believe that Jesus fulfilled that promise.