What are rebuttals to the argument that Christians only worship God to give their lives meaning?
God exists. It is an objective fact with many proofs.
That God created the Universe out of nothing is another objective fact.
God is wise and powerful.. he must be, he made this universe.. and is my Creator. He made me for a purpose. He made me, so I belong to him whether I acknowledge it or not:
The earth is the Lord's and everything in it, the world, and all who live in it: for he founded it on the seas, and established it on the waters. (Psalm 24:1-2)
My duty must be to find him and the purpose for which he made me. As someone has already said
God made men that they should seek him, if perhaps they might feel after him, and find him, though he is not far from every one of us.
My duty must include to worship him who is my maker and who is so great, powerful and wise. Finding him, how could I fail to worship him?
But how can anyone find him? Seeing he is so wise, he must also be patient to allow mankind to live lives ignoring him and lived in rebellion against him.
Seeing he is patient he must be good. He must then have provided a way for us to come to know him.
Looking inwards, I find myself to be often living for myself and not for him. I find this is the spirit of the age, the spirit of the world.. everyone I know is self-seeking. We seek our own glory and ease. We are proud, deceitful, loving the things of pleasure, survival and reputation.
God in his grace leads me to his word, the Bible. I find there a message, the only message, that can provide a way back to God for someone so helpless and sinful as me. It is a way that does not depend on my ability, my uprightness, or power.. it takes for granted I am a needy, helpless, sinner.
The Bible tells me life has meaning because it does not end at death.. it tells me God is perfect in holiness and has appointed a Day when he will judge the world in perfect righteousness and holiness.. there will be a recompense for all sin. This echos in my heart as the truth. Surely life is not meaningless?
But the Bible also tells me God loves me and wants to forgive me and give me a new life. It shows me a mechanism, a way by which a good, perfect God - who hates all sin with an infinite, holy hatred - can forgive a bad person like me. It tells me he gave his only begotten Son to die for my sins, to take the punishment I deserve: in short, to die for me!
And then he rose from the dead to prove his death was a full payment, to prove he is who he said he was, the Son of God.
The truth is the message of the Bible is such great news for a sinner like me that I want to believe it even before I know for sure it is true.
There is no other message in the world like this. All other religious teaching fails to acknowledge the depths of man's lostness and helplessness. They all suppose man can get back to God by efforts of their own.
Receiving his pardon by believing in Christ and what he has done for me and worshipping God through Jesus Christ I receive his Holy Spirit, the gift he promises all his people. And so I enter a new life in which I know my faith is real. I now know it is the truth because my experience is just like what is promised in the Bible.
In short, a Christian worships God not because it gives a subjective meaning to life, not because he has found a truth which appeals to him (though of course it does appeal to him very much) but because he has found The Truth, the objective truth of the meaning of life, and the true way back to God his Creator and Redeemer. The life he now lives he lives by faith in the Son of God "who loved me and gave himself for me." (Galatians 2:20).
Seeing as what I have written so far is a summary of the Gospel, my answer to your question is: present the Gospel. It is the power of God for the salvation of all who will believe it. The questioner gives you on a plate a perfect opportunity to share the essentials of your faith. So follow our example, the apostle Paul: I determined to know nothing amongst you except Jesus Christ and him crucified, and who was crucified, why he was crucified, and what should be our response.