Exodus 19:5 - Wesley's Explanatory Notes

Then ye shall be a peculiar treasure to me — He doth not instance in any one particular favour, but expresseth it in that which was inclusive of all happiness, that he would be to them a God in covenant, and they should be to him a people. Nay you shall be a peculiar treasure: not that God was enriched by them, as a man is by his treasure, but he was pleased to value and esteem them as a man doth his treasure; they were precious in his sight. He took them under his special care and protection, as a treasure that is kept under lock and key. He distinguished them from, and dignified them above all people, as a people devoted to him, and to his service.