OK, I'm back.
Just quick.
Because of Adam's sin, he being the Federal Headship of all humanity, caused the fall.
The fall of man is due to Adam disobeying God. Genesis 1:1 to 3
This fall caused an effect on all of mankind.
We are not personally responsible for his sin, however, we are all affected by it.
This affect is called the sin nature, the nature of sin, original sin, or the flesh.
Original sin is a little different...that would be Adam's sin, but you will hear it used.
So man is born with this sin nature and his tendency is to fail God all the time and so Jesus was necessry
for our salvation.
This man on YouTube explains this very well.
I don't know who he is, that's not why I'm posting it,
I just like how he explains it.
Once you understand him, you might want to listen to someone all Christians respect.
He has a PhD in Philosophy and Theology and is world-reknown and his theology is simple anc biblical.
I'll post him second. His name is William Lane Craig. You could listen to anything he says with no problem.
So here's the first guy:
And here's Craig: