2 Corinthians 9:15 (ESV) … “Thanks be to God for his inexpressible gift!”
Paul ended his appeal for giving with fervent praise to God. The source of all this—the ability to give, the desire to give, even the reconciliation that would occur between Jewish and Gentile believers—was solely from God’s hands. God is the ultimate Giver.
This verse may be saying that the whole process from giver to recipient is an indescribable gift from God. But since Paul used the Greek word for “gift” that is commonly used for Jesus’ gift of righteousness (see Romans 5:15), Paul certainly was thanking God here for Jesus’ gift of salvation. That God freely saves all those who believe in Jesus is truly an “indescribable gift.” God’s extraordinary gift of salvation should motivate you to give generously to others. Spend time meditating on how much God has given you. Then evaluate your generosity in light of God’s generosity to you.
Thankfulness puts everything in the right perspective; God gives what is needed for service, comfort, expression, and recreation. Thankful people can worship wholeheartedly. Gratitude opens our hearts to God’s peace and enables believers to put on love. To increase your thankfulness, take an inventory of all you have (include your relationships, memories, abilities, and family, as well as material possessions). Use the inventory for prayers of gratitude. Before worship, pause and reflect on reasons for thanks. Celebrate God’s goodness. 
 Barton, B. B., & Osborne, G. R. (1999). 1 & 2 Corinthians (p. 411). Wheaton, IL: Tyndale House.