“And when they bring you before the synagogues, and the rulers, and the authorities, do not be anxious how or what you shall answer, or what you shall say, for the Holy Spirit shall teach you in that very hour what you ought to say.”
Having made clear that death might await some of His disciples (Luke 12:4), which would clearly indicate that many of them might expect to be brought before courts for His sake, He now comforts them in the light of the thought of that eventuality. If they are brought before the synagogue, which had religious jurisdiction, or before rulers and authorities, such as Herod or Pilate, then they need not fear that they will not know what to say. For in that hour the Holy Spirit Whom they have received (Luke 11:13) will teach them what to say. Unlike the unbelievers at the last day, they will not be left speechless and comfortless.
Note the contrast. On the one hand are those who blaspheme against the Holy Spirit by finally closing their minds to the Christ of God, and on the other are those who, having responded fully to Him, have the Holy Spirit there as their friend and defender. And in between are those still having to make a decision.
Note the irony of His words. When they are brought before synagogues (the places where the Holy Spirit should be proclaiming His word) the Holy Spirit will give them their words to say in their defence. As a result of His coming the world is being turned upside down.