Similar in meaning, but slightly more formal:
* Truly: This emphasizes the truthfulness of the statement.
* Honestly: This emphasizes the speaker's integrity and lack of deception.
* Verily: An archaic word emphasizing truth and certainty.
* Authentically: This focuses on genuineness and originality.
Similar in meaning, but more casual:
* Honestly: (Again, but more casual)
* Really: This is often used to express agreement or emphasize a statement.
* For real: A more informal way to emphasize truthfulness.
Words that express similar sentiment, but not exactly the same:
* Warmly: This emphasizes affection and goodwill.
* Heartily: This emphasizes enthusiasm and sincerity.
* Respectfully: This emphasizes politeness and consideration.
It's important to choose the word that best fits the tone and context of your writing or speech.