Verses with the word thankful in the New Testament (2 verses):

Romans 1:21
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.

Colossians 3:15
And let the peace of God rule in your hearts, to the which also ye are called in one body; and be ye thankful.