NOTES for Mat 6:1-15
Every action must have a definite goal, and more than that, this goal is reached: the one who loves for show, in order to earn respect and honor, easily reaches his goal - he "has already received his reward." But the one who loves in secret, even from himself, so much that his "left hand does not know" about it, that is, inwardly, in the depth of the heart where even a person's own consciousness does not always penetrate, all the more receives "repayment," that is, reaches his goal.
And it seems that the Gospel, the good news, consists precisely in this promise that the goal will be reached, even if our left hand never finds out about it. Christ promises that compassion, participation in another person's life, will reach that person's heart, that the one who seeks God will manage to be alone with Him in his closet, behind a closed door.