Blessed Theophylact says:
"He exorts His disciples to spiritual sobriety. "Take heed" He says, "what you hear," let nothing of what I say slip away from you. For "with what measure ye mete it shall be measured to you;" that is, whatever degree of attention you give Me, by the same degree will you receive benefit. For if a listener pays the utmost attention, God will give him in return great benefit. But if he is lax, the degree of benefit will likewise be less. He who has eagerness and zeal will receive benefit. But from him who does not have eagerness and zeal, even what he thinks he has will be taken away. Even the small spark of zeal which he used to have is extinguished by laziness, just as it is kindled by attention."
Christ is calling us to spiritual maturity- we are to "act our age". It's OK to have a fifth grade understanding of God if I am in the fifth grade, but if that same understanding of the Faith is what I carry on into adulthood and I do not put the effort in to spiritual growth (mete out a larger measure), the minimal understanding and Faith I have will be taken away.