ייִדישער מחשבֿה אויף דברים 29:28

But for the duties of the heart, there can be no hypocrisy, nor expectation of honor, respect, or praise on account of fulfilling them since other people cannot look into one's heart. Therefore one's intent in fulfilling them is automatically only to the Watcher, namely, the blessed Creator alone, as written "I, the L-rd, search the heart, examine the reins.." (Yirmiya 17:10), and "the hidden things belong to the L-rd our G-d" (Devarim 29:28).

But the prophet refrained from giving an answer because each specific case has its own particular reason (there is no general answer which includes everything - TL). Therefore Moshe Rabeinu commented on this in the torah saying (Devarim 29:28) "the hidden things belong to G-d"..and the wise man said in connection to this "If you see the oppression of the poor, and perverting of justice and righteousness in a province, marvel not at the matter" (Koheles 5:7), and the verse says: "the Rock, His deeds are perfect for all His ways are justice" (Devarim 32:4).

The proof for this view is as follows. G-d has guaranteed to His people that for their visible service, He would give them visible and swift reward in this world. This is explained in parsha Bechukosai "If you will go in My ways.." (Vayikra 26), and likewise, for visible sins, visible and swift punishment in this world, because the masses understand only that which is visible not that which is hidden, as written: "the hidden things belong to G-d, but the revealed things belong to us and to our children, forever" (Devarim 29:28). And the verse says "if the people will turn their eyes away from the [evil] acts of this man and his family, I will turn My face to this man and his family" (Vayikra 20:4). Hence, the reward and punishment for the fulfillment or transgression of the duties of the heart belongs to the Creator. Therefore, Scripture omitted an explanation of their reward and punishment in the next world. (see Tov Halevanon for more details)