I was recently asked this question by a young Christian who is trying to reach his lost friends with the Gospel. What you can read below was my answer. It is divided into two parts. The first section is for those who question the existence of God. The second is for those who believe that God exists, but they question the validity of the Christian God. I hope this helps. Please let me know if you have any other questions!
Arguments for the Existence of God:
1. Argument from Creation - It is impossible that everything just popped up out of nothing. Natural matter can't produce itself - it can only reproduce itself. The natural world (and everything in it) required a supernatural Creator. That Creator (God) did not require a creation himself, because He is not natural - He is supernatural (not bound by the limits of space, time, and matter like we are).
2. Argument from Design - The created order is very intricately designed. The design of the universe implies the existence of a Designer. That Designer is God. Consider a wristwatch. It is very intricately designed with springs, wheels, short/long hands, etc. Did all those parts just come together by themselves? Of course not. The design of the watch requires the existence of a watchmaker. So the very intricately designed universe requires the existence of a Designer (God).
3. Argument from Human Morality - Many people believe in evolution. The theory of evolution is based on the ideas of natural selection and survival of the fittest. If evolution would require that we eliminate the weakest members of our species, then it would make since to go ahead and kill the terminally-ill, the handicapped, the disabled, etc. But for some reason humans go to great pains to save the lives of the weakest among us. Why? Because even our conscience understands that we are made in the image of God (Gen. 1:26). That sense of morality comes from God no matter what the teachers of evolution may tell us (Rom. 2:14-15).
Arguments for the Christian God
1. The Consistency of the Bible - The Bible includes the work of 40 different writers over a period of about 1500 years. Yet the message of the Bible is consistent throughout - humans are sinners in need of a Savior. God has made perfect provision for our salvation through the blood of His Son Jesus who died on the cross to set us free from our sins. If we will simply place our faith in the finished work of Jesus (death, burial, and resurrection) we can be totally forgiven for our sins, experience an amazing life on this earth, and inherit an eternal home in heaven.
2. The Accuracy of the Bible - The historical accuracy of the Bible is amazing. Everything from the location of ancient cities and cultures, the reign of kings and their kingdoms, fulfilled prophecy - all of them point to an amazingly accurate Word of God. The Bible is true in all that it affirms.
3. The Changed Lives of God's People - Every believer in Christ has a story to share. Paul shared his story with King Agrippa in Acts 26 - what his life was like before Christ, what happened when he trusted the Lord, and what God did with his life following his conversion. So moved was Agrippa that he said, "you almost persuade me to become a Christian" (Acts 26:28). I'm sure the pagan king never thought he'd hear those words come off his own lips. No one can deny the work of transformation that Jesus has done in our lives.
