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From the Westminster Shorter Catechism:
Q. 45. Which is the first commandment?
A. The first commandment is, Thou shalt have no other gods before me.119
Q. 46. What is required in the first commandment?
A. The first commandment requireth us to know and acknowledge God to be the only true God, and our God; and to worship and glorify him accordingly.120
Q. 47. What is forbidden in the first commandment?
A. The first commandment forbiddeth the denying, or not worshiping and glorifying, the true God as God, and our God; and the giving of that worship and glory to any other, which is due to him alone.
Q. 48. What are we 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.
Recommended sermon on the First Commandment: The First and Great Commandment, by Pastor Charles Spurgeon (1834-1892)