The Bible makes it real clear in Genesis chapter 2 and chapter 3

That God created Adam, the first man

Then out of Adam, He made Eve

And God and man in a perfect relationship

No problems, no pain, no suffering

No beef, you know what I'm saying?

But God told Adam not to eat from the tree of the knowledge of good and evil

Coz He said the day that Adam does that, he was surely gonna die

But the serpent was the most cunning creature of them all

And he deceived Eve into eating from that tree

And then Eve gave to her husband Adam

And that's when sin entered into the world

And Adam represented all of us, all of humanity

Past, present and future

And we fell short of God's standard, perfection

That's why we call it the Fall of Man

And that's why our world is fallen