Likewise, He saith: “Such are the verses of God: with truth do We recite them to thee. But in what revelation will they believe, if they reject God and His verses?” If thou wilt grasp the implication of this verse, thou wilt recognize the truth that no manifestation greater than the Prophets of God hath ever been revealed, and no testimony mightier than the testimony of their revealed verses hath ever appeared upon the earth. Nay, this testimony no other testimony can ever excel, except that which the Lord thy God willeth.
Here we are at the third quote that Baha'u'llah is using from the Qur'an to help prove the Cause of the Bab. He has already shown us how Muhammad directs us to the Word of God when looking for proof, and how nobody else can write a Surih like it. In this third quote He points out to us that if we reject the verses of God, we have nothing left in which to believe.
In other words, in the previous quote He said that the verses of God were the highest form of writing possible. If you doubt it, try writing something similar. It's impossible.
And if we don't believe in these verses, what are we believing in? Something less worthy? That seems very silly.
Looking at this quote again, one thing we often find helpful is to see the quote in context. Oh, and if you do that, you may discover that different translations put this as verse 45:6, instead of 45:5 as cited in the Iqan. This just has to do with the different translations of the Qur'an.
If we look at the previous verse, we see that Muhammad is referring to the cyclical nature of day and night. This cyclical nature of reality has been an on-going theme throughout the Iqan, demonstrating for us over and over again how to recognize the new Manifestation. Citing this verse, as we have often seen in the Iqan, is a reminder of the previous verse and the context in which we find the quote.
It seems to be reminding us that we have just come through a period of darkness, an epic length night season, and are now back in the light with the new Revelation.
Following the quote Muhammad then refers to those who hear the Word of God and deny it, usually through arrogance, just as Baha'u'llah has mentioned throughout this book.
With this quote, Baha'u'llah is forcing us to make a choice. After reading the verses from the Bab, and recognizing that they are of a comparable quality to the Qur'an, are we going to deny them and find ourselves in the position of the verse after the one cited? Are we going to attempt to take refuge in our arrogance? Or will we acknowledge the superiority of the Writings?
But anything else we want to say is much better said in the next paragraph where Baha'u'llah talks about this very passage.