If we earned eternal life, we would only gain eternal life by our own good deeds thereby being thankful to ourselves and our own ability. If we earned victory over evil we would be thankful for our own power and strategy to conquer the enemy. If we earned favor in the eyes of God we would credit our own wisdom and work in gaining that favor. But the scripture says that there is great thanksgiving and God gets the glory. Why? Because grace comes from what Jesus has done, not from what we have done. Therefore ALL thankfulness goes to Him and He gets the credit for what we have received.
Note that Paul DID NOT say 'as God’s wrath reaches more people, or God’s judgment reaches more people, or God’s punishment reaches more people’. Paul say’s ‘as God’s grace reaches more and more people there will be great thanksgiving’. Who would be thankful for wrath, judgment, and punishment…..except for the self-righteous? It is up to the church to extend the message of grace to more and more people. The church is the messenger of His gift of undeserved favor and blessing. As we extend that message to our friends, family, and nations we help bring forth thanksgiving and glory to the Father. And…..we rejoice with Him!