Lesson of the 1st Born

"It is unsafe to cherish doubt in the heart even for a moment. The seeds of doubt which Pharoah sowed when he rejected the first miracle were allowed to grow, and they produced such an abundant harvest that all subsequent miracles could not persuade him that his position was wrong.... and his heart became more and more hardened until he was called to look upon the cold, dead faces of the first born."
Testimonies for the Church vol.5, p.274 E.G.W.
And it came to pass, that at midnight the LORD smote all the firstborn in the land of Egypt, from the firstborn of Pharaoh that sat on his throne unto the firstborn of the captive that was in the dungeon;
Exodus 12:29