If someone handed you a check for a million dollars, would you casually say “thank you” and move on? I think most of us would have a bigger response than that. After we made sure it was real and acknowledged that we didn’t deserve it, we would be more elaborate in our thanks. We would look for ways to show kindness back to that individual. We would want other people to know how generous and kind this other person was. While we could never repay the person (and if it was truly a gift, they wouldn’t want repayment), we would be intentional about recognizing, appreciating, and finding ways to do something kind for that person. We would want to be sure the individual knew we were using the money in a useful, wise way.
This should be the response of our thanks to Christ for his surpassing gift of eternal life. If we would have a big reaction to someone giving us a lot of money, that will pass away in a few years, how much more should we respond when someone has forgiven us everything and given us eternal life? Saying thanks is important to God, but living a life of thanksgiving is even more important and more valuable. Regularly recognize where everything you have came from. Show Him you love Him by using the things He gave you in a wise way. Honor Him with all you have. Tell others what He has done for you. That is how you say thanks.