Who are God's true people? Are they those who attend any church?

The answer is 'No'. God showed the clear standard in the Bible, so we should look into it.

God said he would distinguish his people through his commandments.

It means even if someone attends a church well, it is meaningless if there aren't commandments.

Then, Where can we keep God's commandments?

It is only the World Mission Society Church of God that keeps the New Covenant in the world.

[Philippians 3:18–20]
For, as I have often told you before and now tell you again even with tears,
many live as enemies of the cross of Christ. Their destiny is destruction,
their god is their stomach, and their glory is in their shame.
Their mind is set on earthly things. But our citizenship is in heaven. . . .

[Exodus 31:14]
“ ‘Observe the Sabbath, because it is holy to you. Anyone who desecrates it is to be put to death;
those who do any work on that day must be cut off from their people.’ ”