You're witnessing to someone. You've asked if he thinks he's good enough to go to heaven, and he's agreed to take the test of the Ten Commandments to find out. He's admitted to stealing something small, and agreed that made him a thief. He's admitted to lying, and agreed that he's a liar. Yep, he blew adultery, too, by having lustful thoughts. And he's confessed to using God's holy name in the place of a filthy, four-letter word. He agrees with you that he's a lying, thieving, adultererous blasphemer at heart.
You press it to the point. Where will God send him on judgment day?

He says heaven.

Why? Because he's a god of love and mercy. His god doesn't judge him or send anyone but maybe Hitler or an ax murderer to hell.

I envisioned that very scene as I read Malachi this morning. Chapter 2, verse 17 says, "You have wearied the Lord with your words. But you say, 'How have we wearied him?' By saying, 'Everyone who does evil is good in the sight of the Lord, and he delights in them.' Or by asking, 'Where is the God of justice?'" (ESV)

Their words weary the Lord. You may grow weary in witnessing and hearing people say such things. Be assured, the Lord Himself is weary of their idolatry as they fashion a god in their own image.

They deny God His holiness. Habakkuk 1:13 says that God is "of purer eyes than to see evil and cannot look at wrong." (ESV) He cannot look at it in the sense that he cannot stand the sight of it. They say God understands they are just human. They say God would not hold that against them. They say that God "loves" them as if that love could mean that he loves their evil. But Psalm 5:4 says, "For you are not a God who delights in wickedness; evil may not dwell with you." (ESV)

They deny God is just. A court of law might rule them guilty, but not God. How foolish. Paul warns in 2 Thessalonians 1:7-9 of the day "when the Lord Jesus is revealed from heaven with his mighty angels in flaming fire, inflicting vengeance on those who do not know God and on those who do not obey the gospel of our Lord Jesus. They will suffer the punishment of eternal destruction, away from the presence of the Lord and from the glory of his might." (ESV)

Perhaps you can use Malachi 2:17 the next time someone wants to deny God His holiness and justice.

On another point, I just wanted to share with you that this afternoon I was in the presence of the Lord. Have you been in the presence of the Lord? It's easy to get caught up in Bible reading, devotionals, and even prayer, and do them thoughtlessly. You can even read your Bible and be truly blessed, pour out your heart in prayer to God. But have you really gotten into his presence.
I don't mean to be mystical about this. It is no magic charm or formula. I'll not say there was anything special I did to get into his presence. But I was alone. I was alone and still and resting. I cannot remember if I was praying or not. But I was suddenly in his presence and it was sweet. I was at once overwhelmed by his Mercy and Grace. I was praying then, but it was not a prayer I was tossing out, but intimate communion. Gratitude poured from me -- gratitude I had not even considered was in me. And, as is the nature of grace, I believe it must have flown from Him, through me, and back to Him.
Spurgeon wrote,

Jesus went apart to hold high fellowship with the Father, and we must enter into the same divine companionship if we would bless our fellowmen. No wonder that the apostles were clothed with power when they came down fresh from the mountain where Jesus was. This morning we must endeavour to ascend the mount of communion, that there we may be ordained to the lifework for which we are set apart. Let us not see the face of man to-day till we have seen Jesus. Time spent with him is laid out at blessed interest. We too shall cast out devils and work wonders if we go down into the world girded with that divine energy which Christ alone can give. It is of no use going to the Lord’s battle till we are armed with heavenly weapons. We must see Jesus, this is essential. At the mercy-seat we will linger till he shall manifest himself unto us as he doth not unto the world, and until we can truthfully say, “We were with him in the Holy Mount.”