Cross reference
1 Corinthians 9:2
2 If to others I am not an apostle, yet at least I am to you; for the seal of mine apostleship are ye in the Lord.
John 6:27
27 Work not for the food which perisheth, but for the food which abideth unto eternal life, which the Son of man shall give unto you: for him the Father, even God, hath sealed.
2 Corinthians 3:1-3
1 Are we beginning again to commend ourselves? or need we, as do some, epistles of commendation to you or from you?
2 Ye are our epistle, written in our hearts, known and read of all men;
3 being made manifest that ye are an epistle of Christ, ministered by us, written not with ink, but with the Spirit of the living God; not in tables of stone, but in tables `that are' hearts of flesh.
2 Corinthians 12:12
12 Truly the signs of an apostle were wrought among you in all patience, by signs and wonders and mighty works.