Thank you for the opportunity to help you with your question!
There are many things which a Christian can tell a believer today about the existence of God. The most effective is: God is real and Christ is real: Instead of thinking about his existence, talking about what others have to say about him, reading about him and so on, Just Test and see if God is real. In the bible it is written " o come and test, see that the Lord is good.." Whatever people hear or speculate about can never outdo, a test.
The second thing they can tell a non believer about the existence of God is letting the non believer understand who a man is. Christians believe that man was created by God in his own image, and their small thoughts they tend to compare God with men. They think he is a old man seated on a throne issuing orders which is not true. God and man are alike. They are both, spirits, they are both invisible, God can not be seen, neither can a man be seen. I know this sounds controversial but it is true that whatever we see is the body of a man but not the man. The man lives inside the body. Now things to do with gender are defined by the body not by the spirit inside(the man) and so we have male and female.
Please let me know if you need further wisdom with this matter
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