Come and hear, all ye that fear God. The address is scarcely to all that have any sense of religion anywhere, as Professor Cheyne suggests, but rather to the religious section of his own nation—the "righteous" or "godly" of other psalms.
They are invited to draw near, and be received into the psalmist's confidence. And I will declare what he hath done for my soul. What God had done for the psalmist was to give him confidence and assurance.
He knew that his prayers would be ineffectual unless his heart was pure. God heard him, and then he became sure that he was free from the "great transgression" (Cheyne).