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.
1 Samuel 3:13 And I declare to him that I am about to punish his house forever, for the iniquity that he knew, because his sons were blaspheming God, and he did not restrain them.
God's name is holy. God's name is his self-chosen revelation about himself and his character. The name of God demands our reverence, our deep-seated awe and respect, and just as God will not tolerate anyone to put any other gods before him, and just as God will not tolerate anyone to worship him in a way that he has not ordained in his word, so too will he not tolerate anyone to abuse and show disrespect to his great and holy name.
Phil Ryken recently wrote:
"The reason God will condemn us (for breaking the third commandment) is because misusing his name is a very great sin. It is a direct attack on his honor and glory, and anyone who makes such an attack deserves condemnation. When people break the third, or any of the commandments, they are guilty before God, and ultimately they will be judged for their sins."
We, of course, know this, and this is why the Christian puts his hope and trust in Jesus Christ's life of perfect obedience to the law of God. What other hope do we have? We know none of us keeps God's law. It is a great grace to be united to Christ, to have his righteousness counted towards us. We must always remember this. But concerning the third commandment, we should also remember that it is a great grace that God would reveal his personal name to unrighteous sinners. It's a great privilege to be able to address the God of all creation by his name, but such a great privilege brings a great responsibility with it. If we love God, if we respect God and care about his glory, we must strive to obey God and show his great and holy name the reverence it is owed.