Romans 1:9
For God, whom I serve wholeheartedly in [preaching] the Gospel of His Son, is my witness [to the truth] of how continually I mention you in my prayers.
Romans 9:1
I am telling [you] the truth in Christ [i.e., since I am a Christian]; I am not lying. My conscience, [confirmed] by the Holy Spirit, supports my statements [as true],
2 Corinthians 1:23
But I appeal to God as my witness [i.e., to verify that I am telling the truth]; it was in order to spare you [i.e., from a disciplinary action] that I decided not to return to Corinth.
2 Corinthians 11:31
The God and Father of the Lord Jesus, who should be praised forever, knows that I am not lying.
1 Thessalonians 2:5
For we were never found using flattering words at any time, as you know, nor did we try to cover up any greedy desires. God can testify [to this].
1 Thessalonians 2:10
You can testify, [just] as God can, how holy, righteous and blameless our conduct was toward you believers.