So what does this word 'sin' mean and why do Christians talk about 'grace'?

Sin is all the stuff we think and say and do that damages our relationship with God. It's kind of like the way in which my relationship with Simone suffers when I think or say or do bad stuff towards her. The thing is, sin has to be dealt with, otherwise the relationship has to end and that breaks God's heart and won't be good for you either!

Grace is a word used to refer to the fact that God takes the initiative. I sometimes find it really hard to begin to be and say sorry to Simone and put things right between us. It would be so much easier if someone else could step in and help me out. Well that's what Jesus did. God knew that we could never do enough to put things right between Him and us, so He sent Jesus to step in and help out. Again, Paul writes:

'Christ arrives right on time to make this happen. He didn't, and doesn't, wait for us to get ready. He presented Himself for this sacrificial death when we were far too weak and rebellious to do anything to get ourselves ready. And even if we hadn't been so weak, we wouldn't have known what to do anyway. We can understand someone dying for a person worth dying for and we can understand how someone good and noble could inspire us to selfless sacrifice. But God put His love on the line for us by offering His Son in sacrificial death while we were of no use whatever to Him.'

That's God's grace. He loves us so much that He let Jesus take the blame and punishment for the sin of everyone.

So how can I possibly respond?!

That's the relatively easy bit! Once you've realised that there's no way that you can have a relationship with God unless someone cleans you up and sorts you out, you can ask Jesus to do that. And He will. Say sorry to Him that He had to go through that for you and thank Him that He did. Ask God to help you to live in a way which will be good for your relationship with Him and help it to grow stronger and deeper. He will send the Holy Spirit to live in you - don't be scared, the Holy Spirit is the 'man on the ground' so to speak. He will nudge your conscience when you're compromising that relationship and will equip you to live a new life with God.