"Although no one can see your accumulated virtues, God will know your good deeds" comes from the Taoist classic "Tai Shang Ying Pian". It means that even if you do good deeds and are not seen by others, you will be rewarded sooner or later. The entire passage of the scripture is as follows:

Although no one can see your accumulated virtues, God knows when you do good deeds. If people are good, even though the blessings have not yet come, the misfortunes are far away; if people are evil, although the misfortunes have not come, the blessings are far away; those who do good are like the grass in the spring garden, which does not grow long, but will be given away every day. A person who does evil is like a sharpening stone. If he does not see the damage, he will suffer losses every day. Blessings and misfortunes always lie in the heart. The terrible thing about doing evil lies not in being discovered by others, but in knowing it. The commendable thing about doing good lies not in the praise of others, but in one's own peace of mind.

The translation is as follows: Even if you do good deeds and are not seen by others, you will be rewarded sooner or later. When people do good deeds, although they have not yet received blessings, disasters are far away. People who do good deeds are like new grass in spring. Although their growth cannot be observed, they are moistened by the sun every day. People who do evil things are like the sharpening stone. Although the wear and tear cannot be observed, there are missing parts every day.

Although you have not encountered blessings and disasters, they are all in your heart. The most terrifying thing about doing bad things is inner demons, and the happiest thing about doing good things is getting married.

Extended information:

"Tai Shang Ying Pian" is one of the classic works of Taoism, which aims to encourage good deeds. It is referred to as "Induction Pian". The author is unknown and the content is a combination of many things. The traditional national thought established a correct image of people in the world, and many of its contents still have positive significance today.

"Heaven will bring blessings to a house that has accumulated good deeds, and misfortune will fall from heaven to a house that has accumulated evil deeds." Summarizes the relationship between karma and retribution, and promotes the ideas of "influence between heaven and man" and karma and retribution throughout the article. It contains both the Confucian ideological and moral character of loyalty, filial piety, friendship and brotherhood, and the Taoist belief treaty of respecting Taoism and virtue. It also includes the Buddhist connotation of compassion, so it can be said that it integrates Confucianism, Buddhism, and Taoism into one place.

Baidu Encyclopedia - Taishang Induction Chapter