As we read between the lines of the Corinthian letters, we can make a very educated guess about Paul's relationship with this Church and why they would make such an accusation. In 2 Corinthians, Paul mentions both a "painful visit" with the Corinthians as well as a "painful letter" he has written to them (now lost to us) indicating the rocky nature of his relationship with this church. Then in 1:16 Paul says "I wanted to visit you on my way to Macedonia and to come back to you from Macedonia and have you send me on my way to Judea." We can gather from what Paul has written that neither of those visits to Corinth took place. Considering the poor state of Paul's relationship with this church, it is easy to see how his failure to follow through on two promised visits would only add insult to injury. We can imagine the Corinthians saying "He visits when he says he won't be here for a while and then doesn't visit when he says he's going to be here. How do we know if we can even trust this guy?".
Paul's response to this accusation is unexpected. If someone accused of me of not being trustworthy my response would likely be to defend myself by trying to demonstrate how I had been consistent even if it didn't appear that way. I would want to explain my rationale for why I did what I did. And for a moment it appears this is what Paul is going to do as well.
"Was I vacillating when I wanted to do this? Do I make my plans according to the flesh, ready to say “Yes, yes” and “No, no” at the same time?"But instead of continuing in that line of thought he begins to discuss the faithfulness of God in Jesus Christ.
"As surely as God is faithful, our word to you has not been Yes and No. For the Son of God, Jesus Christ, whom we proclaimed among you, Silvanus and Timothy and I, was not Yes and No, but in him it is always Yes. For all the promises of God find their Yes in him. That is why it is through him that we utter our Amen to God for his glory. And it is God who establishes us with you in Christ, and has anointed us, and who has also put his seal on us and given us his Spirit in our hearts as a guarantee."Where we would expect Paul to say "Trust me! I know what I am doing!" he essentially says "Trust God because you know what God has already done among you!". Where we expect Paul to talk about his faithfulness he instead talks about God's faithfulness. But this is not a mere distraction on Paul's part. He is not simply dodging a difficult question. He is asking the Corinthians to reconsider how they judge his trustworthiness.
The fundamental question here is whether or not Paul's word can can be trusted. Paul's answer: "Just look at the impact my words have had among you and decide for yourself." A more literal rendering of v. 18 is especially helpful in seeing this: "God is faithful because our word to you has not been Yes and No." God's faithfulness has been exhibited in Paul's words and the Corinthians response to those words. They have come to believe in Jesus Christ in whom all the promises of God find their fulfillment, their "Yes". God has established the Corinthians in Christ and they have received the Holy Spirit because of Paul's preaching. Instead of pointing to his own ability to keep his promises, Paul has pointed to God's ability to keep God's promises through Paul's preaching. God has given Paul's word the ultimate stamp of trustworthiness, far greater than Paul could ever give himself, because of what God has done through that word among the Corinthians. Paul is essentially saying "What more validation do you want?".
In short, Paul is turning the question upside down. The Corinthians want to determine the truth of Paul's words by treating them as an object to be dissected, analyzed, and evaluated against their own standards of truth. But Paul says the real test of his words' truth is not what the Corinthians can do to his words but what his words have done to them ... and by the grace of God those words have saved them.
Perhaps this has profound consequences for the ways we think about Truth as well. Like the Corinthians, we often think of Truth as an object to be discovered and dissected. We evaluate statements,beliefs, and practices based on our understanding of what is true. Obviously, there is some need for this kind of interaction with Truth in our lives. But maybe the greatest truths aren't the ones we can hold at arm's length to be dissected and evaluated from a distance but the ones that get inside us and dissect and evaluate us. Maybe Truth is not primarily about what we know. Maybe Truth is something that happens to us like it happened to the Corinthians. Truth is whatever sets us free.