فَإِن زَلَلْتُمْ مِّن بَعْدِ مَا جَاءتْكُمُ الْبَيِّنَاتُ

Then if you slide back after the clear signs have come to you,

meaning, if you deviate from the Truth after clear proofs have been established against you.
 
فَاعْلَمُواْ أَنَّ اللّهَ عَزِيزٌ
then know that Allah is All-Mighty,

in His punishment, and no one can escape His vengeance or defeat Him.
 
حَكِيمٌ
 
 

All-Wise.

in His decisions, actions and rulings.

Hence Abu Al-Aliyah, Qatadah and Ar-Rabi bin Anas said,

"He is Mighty in His vengeance, Wise in His decision.