When you consider the rebuke that the Corinthians are about to receive throughout the letter it's to some degree surprising that Paul is so thankful for them. However it seems as though what Paul is thankful for in the Corinthians is the grace that is given to them by God in the midst of their waywardness. I believe regardless of the status of who Paul is he still recognizes his deep dependence on these trues in his own life, because whether or not you are living righteously or unrighteously the fact remains that God never changes. He remains true to his promises, and as Paul approaches this rebuke he reminds his friends of this truth as well. Paul describes 5 fundamental principles in the grace granted towards the Corinthians as well as all believers.

1) We are enriched in every way (speech and knowledge) through Jesus Christ.

(I believe this speaks to God's grace opening a believers eyes to the truth of the gospel and allowing them to know Him. This knowledge is also brought to light in His Word. i.e. 2 Timothy 3:16-17)

2) We lack no spiritual gift as we wait for the Lord to return.

(There should be no follower that says they have nothing to offer back. The question is not whether or not we have a the gift but whether or not we are going to use the gift for His glory.)

3) Jesus will sustain us to the end.

(Our hope is not in flesh in blood or ourselves but in Christ alone the hope of glory.)

4) We will be declared guiltless at his return.

(God does not father his children with guilt and shame, but with love and mercy. His righteousness brings forth conviction but at the return of His son the promise of His righteous blood will stand in the midst of our unrighteousness and declare us holy and approved sons and daughters.)

5) God is the faithful one who brought us into relationship with His son.

(We were never good enough to bring grace upon ourselves, or to declare ourselves worthy even if after grace we fail at times.

1 Corinthians 1:1-9