He said this was not just an issue for Christians, it's about all faiths.
"If you take Jesus Christ out of the prayer then nobody knows which God you're talking about. So the Christians have been excluded and all other faiths have been marginalised."
He said the protest tomorrow won't just be Christians.
A group of Muslims plan on attending as well, according to Smith, as by removing the reference to Jesus, Mallard had "reduced God to nothing," and made the prayer too religiously ambiguous.
"It's clear there is an agenda for the ultimate removal of God and religion from our Parliamentary proceedings and this must not happen."
But Mallard's spokeswoman said the prayer was now "more inclusive to all religions".
"The Speaker respects the views of those who are protesting at tomorrow's rally, and had no plans to make further changes to the Parliamentary prayer."
Smith was not convinced Mallard's decision was more inclusive to all religions.
"Over the last few months we've met with numerous Christian groups and churches around the country who are incensed at what is a deliberate step towards dismantling and removing prayer, religion and God completely from our Parliament and nation," Smith said.
He added that New Zealand was founded on Judean and Christian values and principles and the National anthem implores God to defend New Zealand.
"Yet the Speaker thinks he is above our heritage and legacy and can undo a central foundation on which our country was built".