Why were the Israelites safe during the tenth plague if it wasn't because they were good?
They were safe because they applied the blood to their door as an act of faith. Their safety did not rest on personal goodness or merit; it rested solely on the blood. Had they refused and not put the blood on the door, they would not have been safe and would have lost their firstborn.
Les explains that the Israelites in Goshen had nothing to worry about or fear, not because of anything they had earned or because of their goodness, but because of the blood on the door. That blood was an act of faith. If they had scoffed and refused to put the blood on the door, they would not have been safe; they would also have lost their firstborn. Scripture gives no indication any Jews were lost, so evidently every Jewish household in Goshen had the blood on the door and was safe. Les parallels this to being under the blood in the New Testament: safe and secure, though it never gives license. They were sinners just like us, yet the blood applied made them totally safe and secure. This exodus from Egypt was God's redemption of the nation.