Contradiction with Exodus 14:14
Zephaniah 3:16 encourages action to not be afraid, while Exodus 14:14 emphasizes being still and not fighting because the Lord will fight for you.
Exodus 14:14: The LORD shall fight for you, and ye shall hold your peace.
Contradiction with Deuteronomy 20:8
Zephaniah 3:16 encourages those who are fearful to not let their hands be weak, contradicting Deuteronomy 20:8, which suggests the fearful should return home.
Deuteronomy 20:8: And the officers shall speak further unto the people, and they shall say, What man [is there that is] fearful and fainthearted? let him go and return unto his house, lest his brethren's heart faint as well as his heart. [faint: Heb. melt]
Contradiction with Isaiah 41:10
Zephaniah 3:16 suggests that there is still reason to fear, contradicting Isaiah 41:10 which commands not to fear because God is with you.
Isaiah 41:10: Fear thou not; for I [am] with thee: be not dismayed; for I [am] thy God: I will strengthen thee; yea, I will help thee; yea, I will uphold thee with the right hand of my righteousness.
Contradiction with Matthew 10:28
Zephaniah 3:16 implies acting bravely against fear, whereas Matthew 10:28 instructs to fear God who can destroy both body and soul, not just external threats.
Matthew 10:28: And fear not them which kill the body, but are not able to kill the soul: but rather fear him which is able to destroy both soul and body in hell.
Contradiction with John 14:27
Zephaniah 3:16 talks about not fearing on the Day of the Lord, whereas John 14:27 speaks of peace bestowed and advises not to let the heart be troubled at any time.
John 14:27: Peace I leave with you, my peace I give unto you: not as the world giveth, give I unto you. Let not your heart be troubled, neither let it be afraid.