Sucking refers to focused reading of Family literature or listening to a lecture with a believing, "hungry" attitude.
The phrase is a loose reference to the following bible verses:
Psalms 8:2. Out of the mouth of babes and sucklings hast thou ordained strength because of thine enemies, that thou mightest still the enemy and the avenger.
Matthew 21:16. And said unto him, Hearest thou what these say? And Jesus saith unto them, Yea; have ye never read, Out of the mouth of babes and sucklings thou hast perfected praise?
Usage: "Are you guys sucking?"