yet, behold, this [rejection] will indeed become a source of bitter regret for all who deny the truth [of God's revelation]

But truly (Revelation) is a cause of sorrow for the Unbelievers.

And lo! it is indeed an anguish for the disbelievers.

And it is indeed a (source of) grief to the disbelievers.

And indeed, it will be [a cause of] regret upon the disbelievers.