"For although they knew God, they neither glorified him as God nor gave thanks to him, but their thinking became futile and their foolish hearts were darkened"  Romans 1:21

It is hard to imagine a real follower of Jesus, an authentic Christian, who is ungrateful. 

A lack of gratitude is so unbecoming of a believer.  It really looks nothing like Christ when we are ungrateful, and usually speaks to our understanding (or lack thereof) of the work of grace that Christ has done in our lives.

To be ungrateful is to not comprehend the cross. 

To be ungrateful is to have no appreciation for grace.  

To be ungrateful is to misunderstand the incredible mercy that flows from a God of justice. 

To be ungrateful is to forget the condemnation that we were under when we entered this world. 

To be ungrateful is to forget what it was like to live in blinding darkness and brutal bondage. 

To be ungrateful is to put ourselves on the throne of our lives, and since the seating capacity in our hearts is 1 that means that Jesus cannot sit there. 

To be ungrateful is to think that we only need Jesus to save our eternal souls, but not transform our temporary lives. 

To be ungrateful is the inability to flesh out humility in real life. 

To be ungrateful is to live feeling entitled........from God, our relationships, our jobs, our churches. 

To be ungrateful is to forget that if we get what we deserve, then we deserve what we get. 

To be ungrateful, well, it's just not a characteristic that an authentic Christian is known by.