The passage
      v18. By itself, this verse is a little unclear. Paul is probably saying something like the following: "God is to be trusted, he does what he promises, he's reliable, and we, his servants, are similarly trustworthy. As far as our plans to visit you are concerned, they were made with the full intention to follow them through." Just as God does what he says he will do, so Paul and his apostolic team are not vacillating "yes no men."
      v19. Paul's own reliability rests on the reliability of the gospel, a message from a reliable God. As to the content of the message, it concerns "Jesus Christ." They preached Christ, "Son of God". The term "Son of God" is most likely used as a messianic title rather than expressing a filial relationship between Jesus and the Father. So, they proclaimed the message of the coming messiah who through his life, death and resurrection, inaugurates the kingdom of heaven and opens its gates for all. Christ is God's divine "yes" to broken humanity.
      v20. This gospel message, which reveals God's promised intentions for broken humanity, finds its completion, its fulfilment, in Christ. This "Yes", this "Amen" ("affirmation", same idea), this fulfilment of all things in Christ, is faithfully proclaimed by Paul and his apostolic team in their gospel preaching ("spoken by us") - a proclamation "to the glory of God".
      v21. So, Paul's reliability rests on the reliability of God. God is doing what he promised he would do in the gospel, and he is doing it through Paul and his team. It is God himself who verifies, confirms (better than NIV "makes" - making firm) the standing of the apostolic team in Christ, as well as those ("you") who hear their message. Yet, not only has God confirmed Paul and his team, he has also "anointed" them, consecrated them, set them apart for service, or better, "makes us share in his (Christ's) anointing"
      v22. As well as confirming the standing of Paul and his team (as well as all those who believe, "you") and consecrating them for service, God also sets "his seal of ownership on" them, i.e., God seals his servants as a person seals a document for authentication. And as well as this, the apostolic team (as well as "you") possesses the "deposit", the earnest, or first deposit of the Spirit, the first deposit of our eternal inheritance (although, "guaranteeing what is to come", is not in the Greek). So, the suggestion that Paul and his team are vacillating "yes no men" is absurd.

An authentic word from God
      During our Christian walk we will sit under the ministry of many clergy. The danger we face is that we may respond like the Corinthians who judged Paul and his apostolic team from a human perspective. If we do this we will inevitably miss out on the Spirit's word-ministry, which is the means God uses to prepare us for eternity.
      Alan Lad dropped a good line in a B grade cowboy movie once. He was accused of hypocrisy when lecturing a friend on some moral issue. His reply was, "who better to preach than a sinner." Indeed! although sometimes, from the sanctity of the pew, we find it hard to engage with the words of a flawed preacher.
      It is very easy to approach the preached word negatively, yet the Spirit can speak to us through the worst sermon. Methodology and style are really not going to inhibit the Spirit's work. Only falsehood will do that and most preachers aren't into lying. In fact, the plain sermon has more potential to speak to us than its entertaining counterpart.
      When Paul defends his integrity he does so by directing his readers to look beyond the mere man to the integrity of the word he preaches and the reliability of the God who empowers that word:
        i] The preacher's message. The gospel of Jesus Christ declares God's wonderful intentions for those who believe. Its realization rests on the integrity of God, not on the capacity of the preacher. God's divine Word is self-empowering, self-authenticating, and needs only to be heard with an open heart to achieve its intended end.
        ii] The preacher's role. The Word ministry is empowered by the Spirit, who confirms, consecrates, seals and who is a first-installment of glory. The significant involvement of God in the reliability of his Word, itself drives the preacher to deliver a reliable word.
      It is therefore, our responsibility to give heed to the preached word.

Greek notes

v18
      pistoV adj. "faithful" - reliable, trustworthy. Either serving as a comparison, "God can be trusted and so can I", CEV, "as surely as God keeps his words", Barclay, or as an oath, "God is my sure witness that my language toward you is consistent, not vacillating", Bruce. Thrall suggests it is an oath, although a comparison seems more likely.
      hJmwn "our" - of us. The plural "our", when the singular "my" would be expected, is possibly epistolary, therefore "my" may be intended. Paul is possibly being inclusive, referring to the evangelistic team that intended to visit.
      o logoV "message" - the word. It is possible that the criticism against Paul extends beyond his unreliability when making travel plans, to the unreliability of his message, namely, the gospel. Yet, it is unlikely that he is defending the reliability of his "gospel/message" here, but rather his own reliability when it comes to keeping promises. Paul is not into making "airy promises", Hering. "Our promise to visit you wasn't a vacillating `yes' and `no'"

v19
      gar "for" - Note position of this conjunction in the Gk. It serves to underline "God", the "trustworthy" one. "For my example is the Son of a trustworthy God."
      tou qeou ... uiJoV "Son of God" - Is the sense of this title here messianic or filial? It is certainly likely that "Jesus Christ, Son of God" is the substance of the gospel that "was preached among you." As to the meaning, it is possible that the message concerns the divine Son who makes us divine in him, although a message that concerns the coming of the promised messiah, "the Son of God", to gather a people before God, is more in line with the apostolic preaching recorded in Acts.
      IhsouV CristoV "Jesus Christ" - Properly in apposition to "Son of God."
      oJ en uJmin di hJmwn khrucqeiV aor. pas. part. "who was preached among you" - the one / who among you by us having been preached. "Who was preached among you through us", Barclay.
      di emou "by me [and ...]" - though, by means of me. The "us" prompts Paul to explain who the "us" is. He properly places himself first in the list seeing that the criticisms are directed at him. "I mean by myself, Silvanus and Timothy."
      gegonen (ginomai) perf. "it has always been [yes]" - it has been [yes]. The perfect tense serving to indicate a past action with present ramifications, "in him `yes' was and continues to be reality", Hughes. That is, in Christ, the divine promise of salvation finds its fulfilment; "in him the sure `yes' has taken effect", Bruce.

v20
      This verse is difficult to handle as a verb must be supplied in both halves (an ellipsis). Paul is simply saying that Christ fulfills all the promises of God, and to this fact, through Christ, either: i] the people of God give ascent (say "Amen") in worship; ii] Paul and his ministry team give ascent in their gospel preaching.
      oJsai "no matter how many" - as many as. A way of expressing "all" without exception. "In him all God's promises find their `yes'", Barclay.
      nai "yes" - The "yes", "no", form of expression can be a little confusing to the reader. Paul is simply expressing the fact that all God's promises are realized in Christ: Christ is the one "who fulfills all the promises of God", TH; "every promise of God finds its affirmation in him", Phillips.
      en autw/ "in Christ" - in him. Probably "Christ", but possibly "God".
      dio kai "and so" - wherefore also. The kai has adverbial force, "for as."
      di autou "through him" - through, by means of him. If the sense of the "Amen" is a liturgical affirmation directed to God in Christian worship, then Paul is making the point that it is expressed through Christ, "we say it through him - through Jesus Christ our Lord", Barclay.
      to amhn "amen" - the amen, it is true! A Hebrew liturgical word said to give ascent to a prayer, or statement of praise to God. The presence of the article indicates a specific "amen", either in worship, "the customary amen", Plummer, or the affirmation of the "yes" realized in the proclamation of the gospel. Probably not "the final amen", Phillips.
      di hJmwn "[is spoken] by us" - through us. The "us" is possibly the Christian congregation in worship; "through our spoken utterance", Martin. Although not widely accepted, it is possible that the "Amen" is but an affirmation of the "yes" expressed by Paul's apostolic team ("us") in their evangelistic work; "Paul's apostolic labour is an outstanding Amen to God and to his glory", Barrett, cf. Thrall. Of course, as noted above, the verb "spoken" is assumed.
      tw/ qew/ proV doxan "to the glory of God" - for glory to God. "So that God will be honoured", TH.

v21
      de "now" - but, and. Serving to establish a logical connection, "and so ..."
      oJ bebaiwn (bebaiow) pres. part. "God who makes [both us and you] stand firm" - the one establishing, confirming, guaranteeing, making sure, making firm, securing. The present tense expresses ongoing action, the ongoing reliability of God's promises in Christ: "is making us and you stand firm." The sense is either of "confirming / verifying" the status of the apostolic team, "along with you", or "securing" our salvation which is "in Christ", lit "toward Christ", expressing relationship with Christ, union with.
      hJmaV "us" - The first "us" is obviously Paul and his mission team (possibly the apostolic band), while the second "us" may include the Corinthians. None-the-less, the Corinthians are, with Paul, confirmed in the same sure promises of God: "we along with you belong to Christ", Barclay.
      crisaV (criw) aor. part. "He anoints" - having anointed. Reflecting the Old Testament image of an anointing with oil as a sign of commissioning, setting apart for a special purpose. Numerous meanings are possible: set apart for / appointed to ministry / to apostolic ministry; set apart for the kingdom; endowed with the Spirit; endowed with the gifts of the spirit. "Commissioned", RSV; "set us apart", TEV; "Chose us", CEV; "consecrated us to his special work", Phillips. It is possible that the anointing is Christ's and that we share it: "and made us to share his [Christ's] anointing."