Translation:

19. They will be of no use to thee in the sight of Allah: it is only Wrongdoers (that stand as) protectors one to another: but Allah is the Protector of the Righteous.

Notes (Tafseer)

4757. That is, in the service of Allah. Ignorant and contentious men are of no use or service to any Cause. The more you seek their help, the more do their ignorance and their contentiousness increase their own importance in their own eyes. Evil protects (or thinks it protects) evil: it has really no power of protection at all, for itself or for others. The righteous seek the protection of Allah, Who can and will protect them.