How would you explain to an unbelieving friend the meaning of Christ's death on the cross? What Scriptures would you use?

I would tell my friend that Jesus did not come mainly to be a good example — He came to die in our place. Man's problem was never a lack of a high enough standard; it was that we cannot meet any standard, and sin stands as a barrier between us and God. On the cross Jesus took the judgment we deserved, paying for every sin, and by His resurrection He opened the way back to God. That is the whole Gospel: Christ died for our sins, was buried, and rose again.