I’m glad that you are thinking about how best to answer your friend’s question and you are right, that is a very broad question! I will try and outline some major points you might want to think about, but there are heaps of other points you might want to bring up.
Firstly, the greatest thing about being a Christian is being in a renewed friendship with God. That is something really amazing considering that the whole world is steeped in rebellion against God and only deserves death and His condemnation. The Christian is the one who has escaped this anger and condemnation and been brought into wonderful relationship with our Creator again. So being a Christian means being in relationship with God.
Secondly, this relationship with God comes through God’s generosity towards us in Jesus. It is not by any effort on our part, but through God’s kindness (Ephesians 2:6-9). And his grace and kindness is given to us in Jesus his divine Son. Jesus is God great gift to humanity (John 3:16), who brings us to God through his death on the cross for us and his resurrection. Our change in status from being God’s bitter enemy to being God’s friend comes simply through believing in Jesus (Romans 3:22-24). So being a Christian means we can rest from striving to win God’s approval because Jesus has done it for us!
Thirdly, having this new relationship with God, a Christian has a new life now. A Christian can now love and follow God. We do that supremely through following Jesus wholeheartedly, in gratitude and submission. Now Jesus is the Master of the Christian’s life and we call him"Lord". And it is not a lonely fight"God gives his Spirit to help us, we have God himself living in us to help us live a life pleasing to Him! So being a Christian means having our whole lives turned around, from fighting against God to living for God, with the help of His Spirit.
Finally, a Christian is one who has a sure and great hope - that of heaven. We know that we will get to heaven, no matter how much we still fail God now. That is because Jesus’ death and resurrection secures God’s forgiveness for us! The entry ticket to heaven has been won for us, by Jesus. So a Christian is one who has great assurance, who doesn’t have to worry about where he is going after death, and who can look forward eagerly and confidently towards heaven.
Ultimately, the Christian is the most blessed, most fortunate being in the world, because we have been given life and hope, freely from the God who loves us, through Jesus Christ. And that is why a Christian would love to tell of what it means to be a Christian, to those who are not. I hope you have a great time chatting to your friends about being a Christian!