Q: What is the reason annexed to the third commandment?
A: The reason annexed to the third commandment is, That however the breakers of this commandment may escape punishment from men, yet the Lord our God will not suffer them to escape his righteous judgment.

Deuteronomy 28:58-59. If thou wilt not observe to do all the words of this law that are written in this book, that thou mayest fear this glorious and fearful name, THE LORD THY GOD; Then the LORD will make thy plagues wonderful, and the plagues of thy seed, even great plagues, and of long continuance, and sore sicknesses, and of long continuance.

Westminster Project - Thomas Watson's Body of Divinity on the 2nd Commandment