On Sunday, about four million Muslims from every nation on Earth gathered in Saudi Arabia, in the desert of Arafah, as part of the Hajj, the greatest annual pilgrimage known to humankind.

These people, contradicting every evolutionary instinct, stood as equals, representing global humanity, and they did it all day, in the blazing heat of the desert. Why?

The word Arafah means "the meeting place". It refers to the meeting place of Adam and Eve. Muslims believe that when Adam and Eve were cast out of the garden, they were separated on this earth. Eventually, God forgave them both, and reunited them at a place since known as the Mount of Mercy in the middle of the desert of Arafah.

A few kilometres away, and as a thanks to God for reuniting them, Adam and Eve built the first house of God on earth, on the site of the black, cube-shaped building known as the Kaaba in the city of Mecca.

Only a few generations after Adam and Eve, humankind rebelled against God, who then destroyed the Kaaba. It was rebuilt many thousands of years later, by the Prophet Abraham and his first-born son Ishmael, on the same site that Adam and Eve first built the house.

This is also why Muslims all over the world face Mecca when it is time for prayers.

On this, the second day of the Hajj – known as the Day of Arafah – Muslims gathered in the desert, some climbing onto the Mount of Mercy, where Adam once stood, to ask for mercy and forgiveness for their mistakes and ours.

Yesterday, on the third day of the Hajj, the four million Muslim pilgrims followed in the footsteps taken by the Prophet Abraham many thousands of years ago.

In the incident also referred to in the Old Testament, God asked Abraham to sacrifice his only son. Muslims believe this to have been Ishmael and not Isaac. Abraham obeyed, and on his journey to carry out God's request, Satan appeared to him on three occasions and tried to persuade him not to obey God. Abraham threw stones at Satan to drive him away.

When Abraham reached the point where he was about to sacrifice his only son, God intervened and asked him not to. A lamb was placed for Abraham to sacrifice instead.

These days, Abraham's journey is marked, and where he was stopped three times, three huge pillars have been erected. Like Abraham before them, the Muslim pilgrims today also cast stones at the pillars, in a symbolic act of defiance against temptation and the whisperings of Satan. Again, like Abraham before them, at the end of this journey Muslims offer an animal sacrifice. The meat is canned by the Saudi authorities and distributed to the poor around the world.

This day is called Eid and it was celebrated by Muslims around the world yesterday, including those in Bristol.

Today and tomorrow, the Muslim pilgrims will return to Mecca and go around the Kaaba seven times. Having done so, they will have fulfilled their own incredible physical and spiritual journey of a lifetime.