{So We will surely cause those who disbelieved to taste...}

{So We will surely cause those who disbelieved to taste} meaning: By Allah, We will surely cause these people who said [those words]—and the use of the explicit noun instead of a pronoun is to indicate the causality [of their disbelief], or [it refers to] all disbelievers, and they [the aforementioned speakers] are included in that category primarily—{a severe punishment} the extent of which cannot be estimated.

{And We will surely recompense them for the worst of what they used to do}

Meaning: the recompense for the evil of their deeds, which are in themselves the worst. The [Arabic] form af'al [superlative] is used here for absolute intensification.

It has been said: He, glory be to Him, does not recompense them for their good deeds, such as aiding the distressed, maintaining family ties, and entertaining guests, because these are nullified by disbelief. The punishment will be either in both abodes or in one of them.

From Ibn Abbas: {A severe punishment} on the day of Badr, and {the worst of what they used to do} in the Hereafter.