Contradiction with Jeremiah 17:9
This verse describes the heart of man as deceitful, which contrasts the assertion in Isaiah 63:8 about God's people not dealing falsely.
Jeremiah 17:9: The heart [is] deceitful above all [things], and desperately wicked: who can know it?
Contradiction with Romans 3:10
This verse states there is no one righteous, no, not one, which contradicts the notion of righteous and truthful people implied in Isaiah 63:8.
Romans 3:10: As it is written, There is none righteous, no, not one:
Contradiction with Psalm 14:3
Psalm 14:3 notes that all have turned aside and become corrupt, which stands in contrast to the statement that God's people would not lie.
Psalm 14:3: They are all gone aside, they are [all] together become filthy: [there is] none that doeth good, no, not one. [filthy: Heb. stinking]
Contradiction with 1 John 1:8
Claims that if we say we have no sin, we deceive ourselves, opposing Isaiah 63:8's implication that the people were truthful.
1 John 1:8: If we say that we have no sin, we deceive ourselves, and the truth is not in us.
Contradiction with Isaiah 64:6
Declares that all our righteousnesses are as filthy rags, contradicting Isaiah 63:8's description of God's people as not false.
Isaiah 64:6: But we are all as an unclean [thing], and all our righteousnesses [are] as filthy rags; and we all do fade as a leaf; and our iniquities, like the wind, have taken us away.