In the beginning, man happily lived under God's rule and provision on a personal face-to-face relationship. Man chose to reject God's authority and was driven out of the presence of God and lost this personal relationship, as well as coming under the judgment of God for breaking His law.
The only one who could do something about the punishment for this rebellion was God as man was unable to meet the requirements to remove the punishment. God could pay the penalty that was required by His law or leave mankind in their predicament. God being a God of Love chose to pay the penalty on behalf of mankind, so He sent Jesus to die in mankind's place. This He did at Calvary.
Thus by His death, Jesus paid the penalty for man's rebellion and removed the barriers that separated God and mankind so they could once again relate to God on a personal basis if they met God's requirements.
They need to believe Jesus is God, and thus able to meet the requirements of God in regard to their punishment. That He died on the cross at Calvary to do this and that all He had promised them would occur, providing they no longer lived in rebellion against Him and God and tried to live as He wants them to (as found in The Bible).
They must understand that this is nothing to do with their ability to meet this punishment, as they are incapable of doing so, no matter what they do, but that it is a gift of God and that Jesus alone can do this. As such what He has done is the only way to be restored to God and what He taught us is the only way to live for God!