John Calvin Commentary Acts 8:25

"They therefore, when they had testified and spoken the word of the Lord, returned to Jerusalem, and preached the gospel to many villages of the Samaritans." — Acts 8:25 (ASV)

And they testified. In these words Luke teaches that Peter and John came not only that they might enrich the Samaritans with the gifts of the Spirit, but also that they might establish them in the faith which they had already received, by approving Philip’s doctrine. For this is what the word testify means: as if to say, that through their testimony, the word of God had full and perfect authority, and the truth was effective, being well attested and authentic.

Nevertheless, Luke also teaches that they were faithful witnesses of God, when he adds that they uttered the word of God. This was, therefore, the sum of the apostles’ doctrine: to faithfully utter those things which they had learned from the Lord, and not their own inventions, or the inventions of anyone else. He says that they did this not only in the city, but also in villages. Therefore we see that they were so inflamed to advance the glory of Christ, that wherever they went, His name was always on their lips. Consequently, the seed of life began to be sown throughout the whole region, after it had been preached in the city.