Why did the Sanhedrin release Peter and John?
The Sanhedrin released Peter and John after further threatening them because they found no way to punish them. The people were glorifying God for the healing of the lame man, and the apostles insisted they could not stop speaking what they had seen and heard.
Les explains that the Sanhedrin commanded Peter and John not to speak or teach in the name of Jesus, but Peter and John answered that they could not but speak the things they had seen and heard, asking whether it was right to hearken unto men more than God. The council further threatened them but released them because they found no way to punish them, since all men glorified God for the healing of the lame man, who was over forty years old. Les notes that religious people can be instruments of Satan to hold back the truth.
So when they had further threatened them, they let them go, finding nothing how they might punish them, because of the people: for all men glorified God for that which was done.
More Questions
Why did the Jewish council let Peter and John go in Acts 4?
The Jewish council let Peter and John go after further threatening them with prison and probably scourging, because the people made it impossible to punish them.