John 15:25

But this cometh to pass, that the word may be fulfilled that is written in their law, They hated me without a cause.

Their law ... stresses the inordinate regard the priests had for the external features of Moses' law; and it was "theirs" in the sense of the affectionate regard they professed for it, while actually denying it by their sinful conduct. Note that the quotation ascribed to "the law" was not from the Pentateuch, thus revealing that the term "law" was a reference to the entire Old Testament. Passages referred to are:

Let not them that are mine enemies wrongfully rejoice over me;

Neither let them wink with the eye that hate me without a cause (Psalms 35:19).

They trust in vanity, and speak lies; they conceive mischief and bring forth iniquity (Isaiah 59:4).

Hated me without a cause ... means "without a just cause." That there was indeed a reason why they hated Christ is plain in John 3:19. Evil is its own sufficient reason for hating truth and righteousness. John never forgot this teaching and made it the basis of his comment on Cain:

Cain was of the evil one, and slew his brother. And wherefore slew he him? Because his works were evil, and his brother's righteous (1 John 3:12).

That the word may be fulfilled ... Even in the dark hours that lay ahead, with all their sorrow, and even in the contemplation of the flood of evil that would engulf him and his followers, the Lord calmly pointed out that nothing strange was happening; all was going according to God's plan; the Scriptures had foretold all that would happen in the dark hours ahead.