The young man gazed at him as if he would have read the secret of his heart.

'I'm afraid that the picture shows the secret of my heart,' said Basil.

I long to possess you, to share every secret of my heart with you.

It can also be defined as knowing the secrets of another person's heart.

I feel I do not know all the secrets of your heart.

He is going to confide the secrets of his heart to us.

Now I will show you the secrets of your heart.

But he made no secret of his troubled soul and angry heart.

He studied her, perhaps coming to understand the secret of her heart.

I might as well assume you know it all, down to the blackest secrets of my heart.