The first thing Jesus himself ever taught me, in the summer of 1975, was that "taking the Lord's name" means becoming a part of God's family, or being converted. From the point of conversion, the newly "born again" person belongs to that nation of believers who belong to God and are called by His name (e.g., "people of the Lord" or "a child of God"); in other words, they have taken His name upon themselves. So, to "take the Lord's name" means simply to become one of His people.
To use another biblical metaphor, to "take the Lord's name" can mean to enter into a sort of marriage covenant with God and become the "bride of Christ". Old Testament Israel was regularly referred to by the prophets as the wife of God. It was a frequent theme in the prophets that in worshiping foreign gods, Israel was unfaithful to her husband (God) and had become an adulteress (e.g., Ezek. 16).
In sum, "to take the name of the Lord in vain" means to become a believer and then to be unfaithful to the Lord in whom you have believed. People often speak, with romantic flair, of "the promises of God". There are even pretty, little books written about God's promises, usually with lovely, flower-covered covers. But the authors of those books are very selective when choosing which promises to write about. Some of God's promises are terrifying, such as the promise He made to believers who "take His name in vain":
"The Lord will not hold him guiltless who takes His name in vain"
In other words, faithfulness to Jesus is required of "the bride of Christ". Jesus made that clear when he said concerning the Final Judgment of any unfaithfulness servant, "The lord of that servant shall come in a day when he looketh not for him, and in an hour that he is not aware of, and he shall cut him asunder, and appoint him his portion with the hypocrites. There shall be weeping and gnashing of teeth" (Mt. 24:50-51). That terrifying judgment awaiting unfaithful believers is also a "promise of God", but such promises don't fit easily in a book decorated with flowers and butterflies, does it?
The whole point is that if we take God's name upon ourselves by becoming His, then He expects us to live according to His will, and "be holy, for He is holy". Entire Christian sects have been founded on the premise that such is not the case, that once a person is converted, he will be saved in the end regardless of how he or she lives in this world. But the truth is that we cannot escape the promise that God made to those who take His name and then do not do His will. Jesus said, "Not everyone who says to me. 'Lord! Lord!' will enter into the kingdom of Heaven, but those who do the will of my Father who is in heaven." Every account of the Day of Judgment found in the Bible tells us that we will be judge "according to our deeds", not "according to what we claim".
You are not "saved" yet. None of us are. You may be converted; you may have taken the name of the lord, but salvation is the reward that Jesus will give in the end to those who not only take God's name but are also faithful to Him after they take His name. As Jesus told his disciples, "Many false prophets will arise, and will deceive many. And because of a great increase in lawlessness, the love of many [for God] will grow cold. But he who endures to the end [in his love for God], the same shall be saved" (Mt. 24:11-13).