God forgave Hester because she could take her letter off. First of all, if God truly didn’t forgive, he probably would have imprinted the letter on her skin or something, possibly putting it on her forehead or someplace very noticeable. However, that ‘twas not the case. She could take it off whenever she felt like it, but felt like she needed to keep it on to show she had sinned. "Heaven would show mercy," rejoined Hester, "hadst thou but the strength to take advantage of it." She had the choice to keep that symbol of sin on her, not God. This proves that God forgave Hester.
God also forgave Hester