3:179 Allah would not leave the believers in that [state] you are in [presently] until He
separates the evil from the good. Nor would Allah reveal to you the unseen. But [instead]Allah chooses of His messengers whom He wills, so believe in Allah and His
s. And if you believe and fear Him, then for you is a great reward.

Dear Brothers, Assalamo Alaikum W.W.

I shall be glad if anyone of you could provide me explanation for the above verse from the Holy Quran. Specific questions on the same are as under:

a) What does separating the evil from the good from amongst the believers mean? Is this a prophecy of a time when believers will have evil amongst them?
b) What does Allah Almighty mean here by saying he will not reveal the unseen?
c) What does it mean when Allah Almighty says that he will choose of His messengers and that the believers are to believe in them?

Thanks in advance!