13 And since we have the same spirit of faith, according to what is written, “I believed and therefore I spoke,” we also believe and therefore speak,
14 knowing that He who raised up the Lord Jesus will also raise us up with Jesus, and will present us with you.
15 For all things are for your sakes, that grace, having spread through the many, may cause thanksgiving to abound to the glory of God.
The quote in v.13, “I believed and therefore I spoke,” is from Psalm 116:10.
Again, the “we” in verse 13 and the “us” in verse 14 are referring to Paul and those ministering with him.
The preacher and his message occupy a large portion of this letter because popular teachers at Corinth were corrupting the Gospel, and their followers were denying that Paul was either an Apostle or a preacher. — Williams, page 900.
The Spirit of faith in the fact and hope of the resurrection, which animated the Apostle in the endurance of these sufferings, was the same Spirit that animated the Messiah in His sufferings for truth [as seen in Psalm 116]. Believing and speaking belong one to the other. The Apostle was certain that he and his Corinthian converts would be raised from the dead and would stand together in the presence chamber of the King (v.14), and therefore, like the Messiah, he testified because he believed. He knew that He Who raised up Jesus from among the dead would surely raise him up also. The distinction between Christ’s sufferings for truth at the hands of man, and His sufferings in Atonement at the hands of God, must ever be preserved. Paul could share the former with Him, but not the latter.
Like his Master he suffered “all these things” for their sakes (v.15) that grace might abound to them, and that they might have a life of joy and thanksgiving; but all the glory of such grace should be wholly given to God and nothing attributed to him. — Williams, page 901.