Summary: God calls upon His people to have a thankful heart and an "attitude of gratitude."
Paul mentions three things that are God’s will for each and every one of His children. In this series, we have considered how it is God’s will that we be joyful (v. 16) and prayerful (v. 17). Now, we will turn our attention to verse 18, where we are told that it is God’s will for each of His children to be thankful.
1. The focus of the thankful Christian - God!
The focus of the thankful Christian is the same which enables us to remain joyful and prayerful - God. As we focus our attention on God, there are at least two things we are reminded of that enable us to be thankful in all things.
A. The relationship that is ours to enjoy.
As I maintain my focus on God and learn to live my life with a heavenly perspective, I know that what truly makes my life worth living, my love relationship with Christ, can never be taken from me, no matter what the circumstance of life!
We can be thankful in all circumstances, because the circumstances of life do not determine the quality of our life, nor do they diminish the quality of our life, for “our life,” as Paul put it in Colossians 3:3 (quickview) , “is hidden with Christ in God." That’s why Paul tells us in Colossians 3:12 (quickview)  that we should “Set our minds on things above, not on earthly things.”
B. The resources that are ours to employ.
Paul tells us that a thankful heart is God’s will for us not because He wants to burden us down with a sense of guilt. Instead, he tells us this because he wants to reassure us concerning His supply of grace. Later on in this same chapter, Paul reminds us that:
By His grace, God always supplies the power we need to carry out His purpose. By His grace, He will supply the strength we need to maintain an “attitude of gratitude.”
2. The faith of the thankful Christian - In all circumstances!
Note that Paul does not say, “for all circumstances.”
There are certainly some circumstances for which we cannot be thankful.
However, our reason for being thankful is not to be found in the nature of our circumstances. Rather, our reason for being thankful is to be found in the nature of our God.
You see, knowing that God’s basic nature is that He is good and loving should encourage me even in times of great difficulty and distress.
The faith of the thankful Christian in the goodness of God is revealed by the way he looks at the circumstances of life.
A. He looks for the goodness of God in all things.
The thankful Christian follows the instruction of Paul in Philippians:
“Whatever is true, whatever is worthy of reverence and is honorable and seemly, whatever is just, whatever is pure, whatever is lovely and lovable, whatever is kind and winsome and gracious, if there is any virtue and excellence, if there is anything worthy of praise, think on and weigh and take account of these things [fix your minds on them].” - Philippians 4:8 (quickview)  (Amplified)
This thought is illustrated well by Matthew Henry, the celebrated Bible scholar. When a thief broke into his modest home and stole some money from him, he wrote this in his journal: