Matthew 5: ⁶ “How blessed are those who hunger and thirst for righteousness!
    for they will be filled. source 

We can hunger and thirst for many things; but, only one thing can we can hunger and thirst for we are guaranteed we will be filled with.
RIGHTEOUSNESS.

GOD’s Righteousness. 
If we hunger and thirst for Him He will surely supply His Righteousness to us. 

He says so right here, Verse 6, Matthew, chapter 5.
Sermon on the Mount.
The only Perfect Sermon ever recorded.