The bold part is a quote from Psalm 95:7-8. We just saw how Jacob quotes Isaiah in verse 2 while referring to the allegory he just read. Here he quotes from Psalms while referring to the allegory. Mixing Old Testament / Brass plates writings into their own, is Nephi and Jacob's special feat.6 Yea, today, if ye will hear his voice, harden not your hearts; for why will ye ?7 For behold, after ye have been nourished by the good of God all the day long, will ye bring forth evil fruit, that ye must be down and cast into the fire?
Sandwiched between the quote of Psalms and Zenos, Jacob asks "why will ye die?". This may refer to the "hewn down and cast into the fire" by Zenos (see Jacob 5:42,46,66). But it is also related to the theme in Psalm 95 that ends with "they should not enter into my rest". Perhaps both, because they both signify spiritual death. Psalm 95 alludes to the children of Israel who died in the wilderness instead of entering into the promised land. This was a physical death but symbolic of the spiritual death that "the fire" in Zenos represents and that Jacob also talks about on multiple other occasions.