The Sentence Diagram Generator allows you to input complex sentences like the one provided and visually represent their grammatical structure. For the sentence "The LORD God, merciful and gracious, longsuffering, and abundant in goodness and truth, Keeping mercy for thousands, forgiving iniquity and transgression and sin, and that will by no means clear the guilty; visiting the iniquity of the fathers upon the children, and upon the children’s children, unto the third and to the fourth generation," you would follow these steps:

1. Input the Sentence: Enter the entire sentence into the generator.
2. Identify Main Clauses: The tool will help you identify the main clauses and their components.
3. Add Modifiers and Phrases: You can add adjectives, adverbs, and prepositional phrases to illustrate their relationships.
4. Visualize the Diagram: The generator will create a visual representation, showing how each part of the sentence connects grammatically.

This process will yield a comprehensive diagram that illustrates the sentence's structure, making it easier to analyze and understand its components.