[cheekopekman]

We don't have the concept of original sin in our paradigm. We believe every human is born with a clean slate. What St Paul taught you is a foreign concept in both Judaism and Islam. Also, sin is a transgression and Adam wasn't the first transgressor. It was a satan named Iblees in our paradigm or Lucifer in Judaism.

What is missing in Christianity is God's forgiving nature. Many verses in the Jewish Bible speak of God's forgiving nature, the most explicit being Ezekiel 18, which actually cancels out the idea of vicarious atonement introduced by St Paul. There is not a single Jewish prophet, including the son of Mary, that taught this vicarious atonement. 

We believe in a forgiving God, who often forgives without needing payment of any kind.

So you believe every human is born with a clean slate and that your God is a forgiving one. The laws of Allah are there to guide adherents, but if they stumble and fall, they have a Celestial being who has the magnanimity to forgive them, so they can wipe the slate clean and start afresh.

By contrast, Christianity teaches that humanity's problem is much more severe. All people are born in a corrupt state and are held responsible for being in that corrupt state. We have inherited a defective "internal operating system" that alienates us from God. No efforts to eradicate or rehabilitate humanity's rebellious nature that is opposed to God will succeed. As the apostle Paul wrote, "All have sinned and fall short of the glory of God" (Rom 3:23).

We face the problem of immunity to holiness by virtue of our camaraderie with sin. Everyone gets tainted by sin but remains free to act according to his disposition. Because the disposition is corrupt, man lacks the moral ability to obey God. Only by grace can such a fallen creature be restored and redeemed. Without that grace, efforts of moral perfection are doomed to failure.

God never owes us grace. And grace is not justice. If God deals with us ultimately on the basis of justice alone, we will perish. Contrary to what you think, God is both forgiving and just. His righteousness demands that He punish sin, but His love motivates Him to offer a way to forgive sinners.

The question remains unanswered: where did evil begin to exist

Satan is the father of all lies lah!  You believe in his lies and you got tricked liao lah! The same lie he told Adam and Eve in the garden that they would not die if they ate the forbidden fruit and both of them died spiritually and physically lah! I'd rather believe in the truth of God's Word lah! You will know the truth and the truth will set you free lah! If the Son sets you free you are free indeed lah!