Text, meaning, and application

What is the practical force of «فَإِيَّاكُمْ وَإِيَّاهُمْ»?

Steadfastness in Faith, Protection from Trials, and Trustworthy Transmission of Knowledge (Reports 1–6)

• An instruction to buy every new book
• A strong caution against deceivers
• A question has been repeated
• The designation of a particular time