So God, in his eternal wisdom, planned for us to live forever.  With him.  In peace.  But of course we fouled it up.  You can blame it on Adam and Eve, but given a choice, you will still sin.  The only thing that is our salvation is the Holy Spirit--God Himself--within us.

Paul said, "If you continue in the faith grounded and settled, and be not moved away from the hope of the gospel, which you have heard, and which was preached to every creature which is under heaven...Even the mystery which has been hidden from ages and from generations, but is now made manifest to his saints: To whom God would make known what is the riches of the glory of this mystery...which is; Christ in you, the hope of glory."  Colossians 1:23, 26-27

Paul calls it a mystery.  How can Christ be in us?  He just is.  Because God designed us to hold His Spirit back in the beginning when he created us.  There is an empty place within man that looks for something to fill it.   We just have to ask Him to come in and clean us up.

Hundreds of years have past since we first began to discover clans and tribes that were hidden in Africa, S. America, and islands.  But as each clan was discovered, they exhibited one common characteristic.  They had all found something to worship.  They instinctively knew that there was something beyond themselves.  A higher being.  Animals don't do this.

"For the invisible things of Him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse:  Because that, when they knew God...they changed the glory of...God into an image made like to corruptible man, and to birds, and four footed beasts, and creeping things."  Romans 1: 20-22

In other words, idols.  God's glory cannot be contained or exemplified by an image or icon.  He is a Spirit.  That is why most churches have crosses that are empty.  He is risen.