I can say in all sincerity that I knew nothing of these plans.

我可以十分坦诚地说，这些计划我一无所知。

I helped you in all sincerity.

我是一片诚心帮助你。

People believed this, in all sincerity.

人们都真诚地相信这一点。

He spoke in all sincerity.

他极其诚恳地说。

In all sincerity, the book is terrific.

我想极其真诚地说一句，这本书简直太棒了。