Ask them how they know that George Washington, Julius Caeser, or any other person from history existed. The answer is that other people said so, and we look at other people who were not members of their group to see if they are telling the truth. For example, Americans in the 1770’s and 1780’s speak of George Washington. We then look to England, and find that English people from the same time period speak of the Revolution and George Washington, so we can conclude that he was real. There are non-Christian’s from the time of the early Church who verify that Christ was real. Also, all of the Apostles, with the exceptions of Judas and John the Beloved, were killed for their faith in Christ. And it wasn’t a quick painless death, Bartholomew was skinned alive when he refused to renounce Christ. Their willingness to suffer horrific deaths for Christ prove that He was real and that they believed His teachings - they believed that He was and is God.
A good way to show them that God is real is to explain some of the complexities of the universe that show that the material world could not have happened by accident. Also, there are only three possible explanations for the existence of the universe: it has existed for all time (it is God), it made itself (not possible), or someone made it (it was created).
Option 1 is not possible because science shows us that matter, time, and space all had a starting point. Option 2 is not possible because something that is not there cannot make itself be there. Option 3 is possible because it makes sense that something that had a beginning, and could not make itself, must have been made by something else. The fact that time, matter and space had a beginning, means that God must be spaceless, timeless, and immaterial. It is the only logical conclusion.
Check out this video by Sean Forrest:
youtube.com/watch?v=GjGvil3z_XI
In response to 4love2God’s comment on faith, I am a little confused about what you mean.

Science, reason, and Faith are not at odds with each other. We can use science to point to the need for a Creator, and use logic to come to the conclusion that a Creator is necessary. We can come to the conclusion that God/Jesus is trustworthy. This enables us to make the leap from understanding something in its entirety to Faith - trusting in God even though we don’t fully understand Him.
Mankind does not create the existence of God. If God doesn’t exist unless we are thinking of him, then before we were created, we wouldn’t be there to think of Him and that means that He wouldn’t be there to create us! In fact it is the other way around. God created us, and He keeps us in existence by constantly thinking of us and remembering us. God doesn’t make mistakes, what He makes He cannot unmake.
Hope this helps! God bless you, PietroPaolo, in the work that you do!
