(The  true followers of God, though they had suffered persecution and even death at the hands of evil men, had received the seal of eternal life in  Chapter 7.  But it gets worse for the non-followers of God from Chapter 8-on.)

Chapter 8 and 9- The bad things that happen (hail, water turning to blood, water turning deadly bitter, sky becoming dark, locusts, ....all correspond to the plagues on the Egyptians in the Book of Exodus Chapters 7-12.  The point:  In the future, as in the past and in the present, though God’s People will endure hardship, God will deliver those who persevere (to eternal life), and evil will be punished.