When the Apostles were arrested in Acts 4 they were force to confront the very people that had Jesus put to death. While some of these people could be swayed according to the theology they held, many of the members of the Sanhedrin were harsh, having their own interest in view. Their judgment was tempered only by the prevailing will of the people, which, if they could influence as was done in the case of Jesus’ crucifixion, strengthened their resolve to have their own desires implemented. Six months to a year ago these same men had Jesus put to death, and now they found themselves wrestling with his movement in the persons of the Apostles.
Peter did not depend upon his own argument for his defense; what he did was put forward the Scriptures and let them speak for themselves. He recalled to the attention of the Sanhedrin the words of Psalm 118, whose application to these events magnified their own error in crucifying Jesus.
The Psalm begins describing Jesus as a Stone. In fact Jesus used the same term to refer to himself in Matthew 21:42, Mark 12:10 and Luke 20:17. To the one he is a Rock of offense (Isaiah 8:14; cp Romans 9:33), but to the other he is a sure foundation stone (Isaiah 28:16). This stone is life and blessing for those who believe, but the destruction and death for those who reject him, including their man-made systems of organizations (Luke 2:34; 20:18; cp. Daniel 2:34, 44-45). This is very clear in Psalm 118, from which Peter quoted. The Lord’s mercy endures forever for Israel, the priests of God and all who fear him (Psalm 118:1-4); there is no salvation in man, so trust only in the Lord (4-9); although all nations had come against Jesus (the Empire in Pilate, the local authority in Herod Antipas, and the religious authorities in the person of the Sanhedrin), yet he will destroy them in the name of the Lord (10-14); the “right hand of the Lord” (Jesus) is exalted and there is rejoicing in all the tabernacles of the righteous (15-16—via the Holy Spirit in the Temple of God); although he is chastened by God, death has not overcome him, because God has heard his prayer and he will enter the gate of the righteous and praise God forever (17-21); salvation comes through the Stone which the builders rejected, and a new day—which the Lord has made—has dawned upon mankind—a day in which the rejected Stone has become the Head or Lord of all (22-24); Blessed is he that comes in the name of the LORD, and we (his Temple) have blessed him out of the house of the LORD; he is God, and we will praise him and exalt him and give thanks to him for his mercy endures forever (25-29).
So, Peter’s defense lay in the word of God, showing that the rulers had erred, and, if they would repent, would obtain abundant mercy (Psalm 118:3; cp. Acts 4:12 the inclusive **we** must be saved). Although they continued to breath out threats of judgment and punishment, Peter and John told them to judge for themselves if it was righteous to obey man or God (Acts 4:19; cp. Psalm 118:8-9). As for the Apostles, they could do nothing but to speak of what they had seen and heard.
The Apostles were released, because a case could not be built against them. The rulers rejected Christ and his offer of repentance and salvation through the Apostles they confronted at this time. The then current viewpoint (still expressed in modern Judaism) was that the nations (Gentiles) needed to be saved and come under the authority of the God of Israel. By and large, Jews believed they had no need to be saved, because they already have God as their Savior (cp. Nicodemus’ difficulty in seeing Jesus’ argument in John 3:1-12).