{Who themselves also shall tell you the same things by word
of mouth} (kai autous dia logou apaggellontas ta auta).
Literally, "they themselves also by speech announcing the same
things." The present participle, as here, sometimes is used like
the future to express purpose as in  3:26  eulogounta after
apesteilen and so here apaggellontas after apestalkamen
(Robertson, _Grammar_, p. 1128). Judas and Silas are specifically
endorsed (perfect active indicative of apostell(935c)) as bearers of
the epistle who will also verbally confirm the contents of the
letter.