Here thomas paine is using simile to emphasize the pivotal matter that one small crevice or crack now would be like (a simile uses "like" or "as") "a name engraved with the <span>point of a pin on the tender rind of a young oak". meaning, carving your name onto a small sapling of a tree would break it wouldn't it? so that symbolizes a fragile state. people would rather usually carve their name onto a stable and big tree.
