When Pilate saw that he could prevail nothing, but [that] rather a tumult was made, he took water, and washed [his] hands before the multitude, saying, I am innocent of the blood of this just person: see ye [to it].
Note 4 at Mt 27:24: This was not only a symbolic custom of the day to show innocence, but it was also a ritual prescribed in the Jewish Law (De 21:6-7 and Ps 26:6).
Matthew was the only writer to record Pilate washing his hands of guilt, just as he was the only writer to record the dream that Pilate's wife had (see note 3 at Mt 27:19). Together, these instances underscore that Pilate did not innocently condemn Jesus to death. Pilate will be trying to wash the blood of Jesus off of his hands through all eternity.
Just as God was faithful to show even Pilate the truth in this situation, we can be assured that all those who have ever rejected Jesus have done it in spite of the conviction of the Holy Spirit in their hearts (Ro 1:18-20).