{Encouraged him} (protrepsamenoi). First aorist middle
participle of protrep(935c), old verb, to urge forward, to push on,
only here in the N.T. Since Apollos wanted (oulomenou autou,
genitive absolute) to go into Achaia, the brethren (including
others besides Priscilla and Aquila) wrote (egrapsan) a letter
of introduction to the disciples in Corinth to receive him
(apodexasthai auton), a nice letter of recommendation and a
sincere one also. But Paul will refer to this very letter later
( 2Co 3:1 ) and observe that he himself needed no such letter of
commendation. The Codex Bezae adds here that certain Corinthians
who had come to Ephesus heard Apollos and begged him to cross
over with them to Corinth. This may very well be the way that
Apollos was led to go. Preachers often receive calls because
visitors from other places hear them. Priscilla and Aquila were
well known in Corinth and their approval would carry weight. But
they did not urge Apollos to stay longer in Ephesus. {Helped them
much} (sunebaleto polu). Second aorist middle indicative of
sunball(935c) used in  17:18  for "dispute," old verb to throw
together, in the N.T. always in the active save here in the
middle (common in Greek writers) to put together, to help.
{Through grace} (dia t(8873) charitos). This makes sense if taken
with "believed," as Hackett does (cf.  13:48  16:14 ) or with
"helped" ( 1Co 3:10  15:10  2Co 1:12 ). Both are true as the
references show.