Have you ever encountered someone who didn’t believe in God because they said there was no proof He existed?  There are people out there who struggle with the whole concept of faith.  They need something concrete to sink their teeth into in order to believe. 
Thomas was that way about Jesus being raised from the dead.  Jesus appeared to some of the disciples, but Thomas wasn’t with them.  In John 20:25 Thomas said, “Unless I see the nail marks in His hands and put my finger where the nails were, and put my hand into His side, I will not believe it.”  This was a man who walked with Jesus for three years and sat under His teaching for three years, yet he still didn’t believe.  In John 20:26-29 we read about Jesus appearing again before the disciples and Thomas was present.  Jesus immediately addressed Thomas’ doubt by encouraging him to reach out and touch the very places he said he’d need to touch in order to believe.  At the end of verse 29 Jesus said, “Because you have seen Me, have you believed? Blessed are they who did not see, and yet believed.”
Psalm 75:1 says, “We give thanks to you, O God; we give thanks.  You are present, and your miracles confirm that.” [God’s Word translation].  All we have to do is look around us in nature and we see confirmation He exists.  The birth of a baby, the seas ebbing and flowing without crossing their boundaries, air flowing in and out of our lungs, the sun rising and setting each and every day…these are but a few miracles we witness on a regular basis that confirm His existence.
Charles Sineath once said, “Never debate the Gospel.”  Those who are trying to draw us into a debate about the existence of God are not seeking Him, they’re seeking an argument.  If they are truly seeking, they may ask hard questions, but they aren’t arguing; they’re searching for Him.  When that happens, you need not worry about what you will say or how you will answer their questions. Mark 13:11b says, “Just say whatever is given you at the time, for it is not you speaking, but the Holy Spirit.”  When we find ourselves in a difficult situation or a place where we don’t know how to respond, we can rely on the Holy Spirit to guide us and direct us.  Remember the middle of Psalm 75:1… “You are present”.  Just like air, God is always there. He is with us, every step of the way, so take a chance and share how awesome He is with those you encounter and trust Him to do the rest.  Remember what Paul said in 1 Corinthians 3:6, “I planted the seed, Apollos watered it, but God made it grow.”  The hard part is up to Him and He is worthy of the task.