Cross reference
1 Corinthians 9:2
2 If to others I am not an Apostle, at least I am one to you: for the fact that you are Christians is the sign that I am an Apostle.
John 6:27
27 Let your work not be for the food which comes to an end, but for the food which goes on for eternal life, which the Son of man will give to you, for on him has God the Father put his mark.
2 Corinthians 3:1-3
1 Do we seem to be again attempting to put ourselves in the right? or have we need, as some have, of letters of approval to you or from you?
2 You yourselves are our letter, whose writing is in our heart, open for every man's reading and knowledge;
3 For you are clearly a letter of Christ, the fruit of our work, recorded not with ink, but with the Spirit of the living God; not in stone, but in hearts of flesh.
2 Corinthians 12:12
12 Truly the signs of an Apostle were done among you in quiet strength, with wonders and acts of power.
