If I be not an apostle unto others, yet doubtless I am to you: for the seal of mine apostleship are ye in the Lord.
Yet doubtless - yet at least I am such to you.
Seal of mine apostleship - your conversion by my preaching (2 Corinthians 12:12), and your gifts conferred by me (1 Corinthians 1:7), vouch for my apostleship, as a seal to a document attests its genuineness (John 3:33).