his sincerity
彼は借金を全部払ったが、それは彼の誠実さの証拠である。
He paid off all his debts, which is proof of his integrity.
このことは友人に対する彼の誠実さを示すものだ。
This shows his sincerity toward his friend.
彼女に彼の誠実さを確信させた。
It convinced her of his sincerity.
彼の誠実さを疑うのなら彼に援助を求めるな。
If you doubt his integrity, don't ask him for help.