Mark 2:7 Why doth this man thus speak blasphemies? who can forgive sins but God only?
The term blasphemy is not often used today, but in the past someone found legally guilty of blasphemy could receive the death penalty. In fact, this is the crime that Jesus was accused of which led to His crucifixion. Mark 14:64 I address this issue in my post Did Jesus Blaspheme?
One of the 10 commandments (Exodus 20:7) says we are not to “take the name of the LORD thy God in vain.” Through this command blasphemy became a legal principle applied to any disrespectful use of God’s name. It soon developed to comprehend all human communication concerning the person of our Creator.
Blasphemy, in its common use in the U.S., is applied to the use of God’s name in verbal profanity. Profanity of this sort is allowed because of the U.S. reverence for freedom of speech and religion. But blasphemy has a much broader application. It applies to any form of communication, including art, which could be considered to be disrespectful to God's person. It is a matter of respect for God, respect for the revealed truth about Him, and respect for His authority over creation.
The word blasphemy also includes the use of God’s name for empty purposes or profit. For example, a politician blasphemes when he uses God’s name for political purposes. A blasphemy, in my opinion, that is much worse than other common forms of profanity. But blasphemy could also apply to the use of God's name by a Christian who says something is the will of God when it is clearly only the will of that Christian. Christians often use God's name to spice up their show of spirituality... but that's blasphemy, and is much more serious than the profanity used at the neighborhood bar.
I don't want to belabor this point, but I do want to raise awareness on this subject. What about speaking for God in the first person when we have no record of God ever having said what He is quoted as saying? Is that blasphemy? I think so. Speaking for God seems to me to be the most presumptuous of all human acts, and is most common among those who appear to be nearest Him. An open, public familiarity with God can create pressures that are most easily released by taking authority and license that is not ours. That God loves us is clear... that believers, through the Gospel, are in a relationship with Him is also clear... but that does not make us as He is... He is still God our transcendent Creator and we must revere Him.