Taking God’s name in vain.

Proverbs 30:7-9 KJV

[7] Two things have I required of thee; deny me them not before I die:

[8] Remove far from me vanity and lies: give me neither poverty nor riches; feed me with food convenient for me:

[9] Lest I be full, and deny thee, and say, Who is the LORD? or lest I be poor, and steal, and take the name of my God in vain.

Taking God‘s name in vain means to claim to be submitted to Him, to claim to obey His commandments. Yet violating them presumptuously, rationalizing disobedience (“I am poor, so I may steal.”).

Presumptuous sins are worse than other sins.

Psalm 19:13 KJV

[13] Keep back thy servant also from presumptuous sins; let them not have dominion over me: then shall I be upright, and I shall be innocent from the great transgression.

Names in the Bible are descriptive. Consider,

Psalm 48:10 KJV

[10] According to thy name, O God, so is thy praise unto the ends of the earth: thy right hand is full of righteousness.

Genesis 11:4 KJV

[4] And they said, Go to, let us build us a city and a tower, whose top may reach unto heaven; and let us make us a name, lest we be scattered abroad upon the face of the whole earth.

So when a person claims to be a Christian, they are describing themselves as submitted to the authority of Christ, with their hearts set on obeying Him. They take on God‘s name. This is a vain or worthless description of themselves, if they disobey Him presumptuously (“If I sin, God will forgive me, so it does not matter.”).