telling them that morality comes from God would probably not be all that convincing.
First, I suggest we get what can help us, ourselves, about this.
One thing is how God our Father knows what is best for us. So, He has rules about what is a no-no so we do not go in the opposite direction. And He knows what things we would be doing in an evil spirit >
"the spirit who now works in the sons of disobedience" (in Ephesians 2:2).
While people are doing wrong things > we can tell we are being deeply degraded emotionally and spiritually and in our thinking and feelings. I can tell, right away, how I am losing something as soon as I get started in even thinking of doing something wrong. We Christian people have the ability to sense inside ourselves, as well as to know.
And we need to understand things which the Bible says, about what is for love and what is anti-love. There are things which are wrong, but the Ten Commandments do not directly name these things. And we need to understand how they are wrong in comparison with how love will have us be in our character and what love will have us doing.
"Let all bitterness, wrath, anger, clamor, and evil speaking be put away from you, with all malice. And be kind to one another, tenderhearted, forgiving one another, even as God in Christ forgave you." (Ephesians 4:31-32)
So, this shows also how unforgiveness is wrong, even though the Ten Commandments and a lot of church and religious teaching on morals might not say a word about forgiving.
And you can feed on Philippians 2:13-16 >
Arguing and complaining can indeed be anti-love, and have us deeply degraded by Satan's spirit which is so anti-love, anti-peace, degrading us from kindness and gentleness and being humble and resting in Jesus.
So, as we grow in Jesus and our knowledge of God's word, our moral standards become deeper than being only about our outward behavior.
So . . . this I would say needs to be in your background, while you are talking with an atheist. So . . . what do you say to the person?
You might say God is your Father, and He knows what is best for you. And you have experienced how better off you are, now that you go by Biblical standards. And note how our desires need to be right, not only our actions, since Jesus cares about our heart and not only what we are showing.
And so, of course, this can bring up if it is ok to have preferences for a wrong thing even if we don't do it. Well, the heart needs to have God's preferences; and so why ever would we want to excuse less????
And I would say stay with this, and do not try to argue or use some other sort of logic. Stay with God being our hope. And since God is so more than we humans are, we need how God reaches us and proves Himself in us.
There are many things humans try, in order to prove or disprove God; and if they have not worked it is because they don't work
God is quiet, not silent; so we need to be still and submissive to Him in His peace, so we can know Him and share with Him and do His will. But many are deeply suffering, and they are desperate to feel some pleasure to make them feel alive, happy, successful, important, and in control. But this is seeking physical pleasure, instead of seeking God for Himself; and so ones can totally miss God because their attention is on pleasures and how they are feeling.
And inside themselves their anger and frustration and demanding what they want is keeping them noisy, instead of quiet.
And Jesus understands this, and Jesus cares about us; so Jesus calls to
"all" who are deeply in trouble >
"'Come to Me, all you who labor and are heavy laden, and I will give you rest. Take My yoke upon you and learn from Me, for I am gentle and lowly in heart, and you will find rest for your souls.'" (Matthew 11:28-29)
Jesus cares about us, and He knows what our problem is; He does not try to debate and argue with us, but He simply tells us He cares about us and knows what we need,
and only Jesus is able to do all that we need.