Indeed
Really, Sincerely
adverb
The に on the end of this word turns sincerity into an adverb. What's the adverb version of sincerity? It's sincerely, or indeed. It really is.
まことに
This shares the same reading as the vocabulary word 誠, but here's a reminder just in case:
My coat (まこと) really is a coat! Indeed, it is.
誠に申し訳ございません。
I sincerely apologize.
わざわざお越しいただき、誠にありがとうございます。
I really thank you for coming all the way here.
それは誠に素晴らしいことだ。
That is indeed wonderful.
誠に申し訳ありませんが、当ホテルは全室禁煙となっております。
I am really sorry, but all rooms in our hotel are non-smoking.