The boldness of Peter and John - Την παρῥησιαν, The freedom and fluency with which they spoke; for they spoke now from the immediate influence of the Holy Ghost, and their word was with power.

That they were unlearned and ignorant men - Αγραμματοι, Persons without literature, not brought up in nor given to literary pursuits - and ignorant, ιδιωται, persons in private life, brought up in its occupations alone. It does not mean ignorance in the common acceptation of the term; and our translation is very improper. In no sense of the word could any of the apostles be called ignorant men; for though their spiritual knowledge came all from heaven, yet in all other matters they seem to have been men of good, sound, strong, common sense.

They took knowledge of them - Επεγινωσκον may imply that they got information, that they had been disciples of Christ, and probably they might have seen them in our Lord's company; for there can be little doubt that they had often seen our Lord teaching the multitudes, and these disciples attending him.

That they had been with Jesus - Had they not had his teaching, the present company would soon have confounded them; but they spoke with so much power and authority that the whole sanhedrin was confounded. He who is taught in spiritual matters by Christ Jesus has a better gift than the tongue of the learned. He who is taught in the school of Christ will ever speak to the point, and intelligibly too; though his words may not have that polish with which they who prefer sound to sense are often carried away.

were.

Acts 2:7-12 And they were all amazed and marveled, saying one to another, Behold, are not all these which speak Galilaeans...

Matthew 4:18-22 And Jesus, walking by the sea of Galilee, saw two brothers, Simon called Peter, and Andrew his brother, casting a net into the sea...

Matthew 11:25 At that time Jesus answered and said, I thank you, O Father, Lord of heaven and earth...

John 7:15,49 And the Jews marveled, saying, How knows this man letters, having never learned...

1 Corinthians 1:27 But God has chosen the foolish things of the world to confound the wise...

they took.

Matthew 26:57,58,71,73 And they that had laid hold on Jesus led him away to Caiaphas the high priest, where the scribes and the elders were assembled...

Luke 22:52-54,56-60 Then Jesus said to the chief priests, and captains of the temple, and the elders, which were come to him, Be you come out...

John 18:16,17 But Peter stood at the door without. Then went out that other disciple, which was known to the high priest...

John 19:26 When Jesus therefore saw his mother, and the disciple standing by, whom he loved, he said to his mother, Woman, behold your son!

Impossible Silence
'We cannot but speak the things which we have seen and heard.' --ACTS iv. 20. The context tells us that the Jewish Council were surprised, as they well might be, at the boldness of Peter and John, and traced it to their having been with Jesus. But do you remember that they were by no means bold when they were with Jesus, and that the bravery came after what, in ordinary circumstances, would have destroyed any of it in a man? A leader's execution is not a usual recipe for heartening his followers, …
Alexander Maclaren—Expositions of Holy Scripture: The Acts

A Manifestation of God in Answer to Prayer
A MANIFESTATION OF GOD IN ANSWER TO PRAYER ". . . When they had prayed, the place was shaken where they were assembled together; and they were all filled with the Holy Ghost, and they spake the word of God with boldness." -- Acts 4:31. This remarkable manifestation of God reveals the effectiveness of prevailing prayer. It vividly discloses the fact that prayer can accomplish much. It is apparent that neither the apostles, nor the assembled company of devout believers were astonished at the marvelous …
T. M. Anderson—Prayer Availeth Much

Against the Modern Free-Thinkers.
Sir, THERE arrived in this neighbourhood, two days ago, one of your gay gentlemen of the town, who being attended at his entry with a servant of his own, besides a countryman he had taken up for a guide, excited the curiosity of the village to learn whence and what he might be. The countryman (to whom they applied as most easy of access) knew little more than that the gentleman came from London to travel and see fashions, and was, as he heard say, a Free-thinker; what religion that might be he could …
Joseph Addison—The Evidences of the Christian Religion, with Additional Discourses