I would tell the unbeliever that all man are sinners and fall short of the glory of God. All men are guilty and deserve to die because of sin.
Because men cannot refrain from sin, for it is their nature to sin, God sent his Son into the world to give man hope. He loved the world so much he sent his only begotten Son, so that whoever believes in him shall not perish, but shall have eternal life.
I would tell him that Jesus came and took the world's sin upon the cross, where it was destroyed.
So that all who ask God for forgiveness can be saved from death.
Jesus said he who believes in me will never die.
So a man who believes in him though he dies in the flesh, his soul/spirit goes to be with Jesus.
Jesus said that he who believes in him, though he be dead, yet shall he live.
We all must suffer physical death. But those who are born again, those who believe will go to be with Jesus upon physical death.
If the soul of a man remains dead in him because of sin, at the point of physical death, because his soul is dead in tresspasses, it is taken to the place where the unrighteous dead are reserved until the day of Judgment.
A man must be born again to enter the kingdom of God. John 3:5
A man must be born again, renewed, made alive again in his spirit and remain so to the point of physical death. And because his soul/spirit is alive it goes to be with Christ in paradise.
When I talk of one being a Christian, I don't refer to any denomination or doctrines of men.
I refer to all who have been born again by the grace of God through faith in his Son. I speak of all those who have been baptized into Christ through his blood and their sins have been washed away. I am referring to all who have passed from death unto life being renewed, born again in their spirits by the quickening spirit Christ Jesus.
So don't put much stock in the doctrines of men for they will claim many a soul in the name of hell.
Believe only the Word of God. Steer clear of those who add or take from the Word of God. Seek the face of the Lord and let his spirit, that spirit of truth known as the Holy Spirit lead you into all truth.
REMEMBER, there will be no denominations in the kingdom of heaven, only the gospel of Jesus Christ.