A.T. Robertson Commentary Acts 18:27

"And when he was minded to pass over into Achaia, the brethren encouraged him, and wrote to the disciples to receive him: and when he was come, he helped them much that had believed through grace;" — Acts 18:27 (ASV)

Encouraged him (προτρεψαμενο). First aorist middle participle of προτρεπω, old verb, to urge forward, to push on, only here in the N.T. Since Apollos wanted (βουλομενου αυτου, genitive absolute) to go into Achaia, the brethren (including others besides Priscilla and Aquila) wrote (εγραψαν) a letter of introduction to the disciples in Corinth to receive him (αποδεξασθα αυτον), a nice letter of recommendation and a sincere one also. But Paul will refer to this very letter later (2 Corinthians 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 (συνεβαλετο πολυ). Second aorist middle indicative of συνβαλλω 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 (δια της χαριτος). This makes sense if taken with "believed," as Hackett does (cf. 13:48; 16:14) or with "helped" (1 Corinthians 3:10; 1 Corinthians 15:10; 2 Corinthians 1:12). Both are true as the references show.