What did Apollos know before he received more complete teaching?
Before receiving more complete teaching, Apollos knew only what took place under John the Baptist's preaching—namely that Jesus was the Christ, the Messiah, the Son of God. He had been instructed by someone else and taught from the Old Testament, but his knowledge did not yet include the fuller way of God.
Les explains that Apollos had been instructed by someone else, so whatever he knew and whatever he taught from the Old Testament came through that instruction. His knowledge was limited to what took place under John the Baptist's preaching—that Jesus was the Christ, the Messiah, the Son of God. He began to speak boldly in the synagogue, and no doubt he had a right to do so: he was intelligent, well taught, and eloquent. After they heard what he was preaching, they took him unto them, probably took him home for dinner after the synagogue service, and as they sat around the table they expounded unto him the way of God more perfectly or more completely.