Q: What are specially taught by these words before me in the first commandment?
A: These words before me in the first commandment teach us, That God, who seeth all things, taketh notice of, and is much displeased with, the sin of having any other god.

1 Chronicles 28:9. And thou, Solomon my son, know thou the God of thy father, and serve him with a perfect heart and with a willing mind: for the LORD searcheth all hearts, and understandeth all the imaginations of the thoughts: if thou seek him, he will be found of thee; but if thou forsake him, he will cast thee off for ever.
Psalm 44:20-21. If we have forgotten the name of our God, or stretched out our hands to a strange god; shall not God search this out? for he knoweth the secrets of the heart.

Westminster Project - See John Flavel on the First Commandment