Emphasis in this verse falls on the words to others and to you. Paul is not thinking objectively about who is or is not really an apostle, but about whether or not the Christians at Corinth have accepted him as an apostle. The contrast between others and you suggests that the others are outside the Christian community at Corinth, or at least outside the group to which Paul is writing. They are probably the same as those “who would examine” Paul in verse 3. The clause If to others I am not an apostle can be rendered as “Even if others do not recognize that I am an apostle” or “… do not recognize that I am a messenger from Jesus Christ.” At least I am to you may be phrased as “I am sure that you recognize that I am an apostle.”
The second part of verse 2, from for you are the seal …, can be analyzed grammatically in various ways. It is clear that seal or “proof” (Good News Bible) means evidence or confirmation that Paul is a real apostle. The problem arises with the last four words, literally “you are in the Lord,” which are the same as the end of verse 1. If they are understood here also as “through the Lord,” the meaning would be “you are my proof of my work as an apostle for the Lord.” Barclay translates “the fact that you are Christians is the seal that guarantees that I genuinely am an apostle.” Translations that follow the order of the Greek, such as Revised Standard Version, New International Version, and New Jerusalem Bible link in the Lord with apostleship; Revised English Bible has “in the Lord you are the very seal of my apostleship.” The parallel with verse 1 is an argument in favor of Revised Standard Version‘s rendering, but it is impossible for us to be certain. A possible rephrasing of this sentence is “The fact that you yourselves live in union with the Lord proves that I am an apostle.”
Quoted with permission from Ellingworth, Paul and Hatton, Howard A. A Handbook on Paul’s First Letter to the Corinthians, 2nd edition. (UBS Handbook Series). New York: UBS, 1985/1994. For this and other handbooks for translators see here .