For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse:  (21)  Because that, when they knew God, they glorified him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened.

Romans 1:20-21 

Explanation:

God, Himself, is not seen through physical eyes.  So how can someone be held accountable for believing in something or someone they can not see?  Paul tells us that the seen world was created by the invisible God.  By simply viewing the natural world we can come to an understanding that there is a powerful and eternal being that exists that had to create what we see.  The Holy Spirit communicates here that this is clearly seen.  God is actively revealing Himself through the created world to mankind.  We know that something before us and more powerful than us had to create this.  Paintings come from painters.  Sculptures come from sculptures.  Cars must have car designers and manufacturers.  This created world, with all of its order, is designed.  It is obvious.  Even many scientists today are coming to this conclusion.  Check out this video.Check out this video.  https://www.youtube.com/watch?v=noj4phMT9OE

The response of most people to this simple fact is not to seek out the true and living God.  Many do not respond to the light they have been given.  They do not glorify the God who made them.  They were not thankful for the common grace that was given to them.  Instead, what they do with the common grace they were given is to use this freedom and grace for themselves.  Their “imaginations”, or what they can think up to do and live for, is empty and foolish. 

Application:

The two things that they did not do in the verses in response to God is glorify Him or be thankful to Him in response to His creation.  We who believe in God, and have been changed by the Gospel, should respond in these two ways to God!  He is worthy of our worship and of our thanksgiving.  We ought to glorify Him for His eternal power and Godhead.  We should be thankful to Him for the things that He has made.

Response:

• Take some time to worship God today through a prayer and/or a worship song giving.  Aim at giving Him glory in this way.
• Pray a prayer with no requests, thanking God for what He has done for you.