What does sufficient to the day mean?

Will we ever be able to penetrate into all the mysteries in this life? 

Do not disturb yourselves about the things of the future—sufficient to the day—now is the time you are to improve and when the time comes for your future lives the good Father will not send you into a void, for there is none where his loving arms cannot enfold you. I do not wish to check you in any pursuit of knowledge, but you must learn the alphabet of things before you can penetrate into the mysteries.

Will we ultimately be able to penetrate into all the mysteries?

Never—all the lives forever and forever will still leave things unknown.

—Insight