Adam Clarke on James 3:9

Therewith bless we God - The tongue is capable of rehearsing the praises, and setting forth the glories, of the eternal King: what a pity that it should ever be employed in a contrary work! It can proclaim and vindicate the truth of God, and publish the Gospel of peace and good will among men: what a pity that it should ever be employed in falsehoods, calumny, or in the cause of infidelity!

And therewith curse we men - In the true Satanic spirit, many pray to God, the Father, to destroy those who are objects of their displeasure! These are the common swearers, whose mouths are generally full of direful imprecations against those with whom they are offended.

The consideration that man is made after the image of God should restrain the tongue of the swearer; but there are many who, while they pretend to sing the high praises of God, are ready to wish the direst imprecations either on those who offend them, or with whom they choose to be offended.