I believe the confusion between grace and law is summed up by two seemingly contradictory verses:
For sin shall not have dominion over you: for ye are not under the law, but under grace. What then? shall we sin, because we are not under the law, but under grace? God forbid. (Rom 6)
And unto the Jews I became as a Jew, that I might gain the Jews; to them that are under the law, as under the law, that I might gain them that are under the law. To them that are without law, as without law, (being not without law to God, but under the law to Christ,) that I might gain them that are without law. (1 Cor 9)
Now, we both know that Scripture does not ever contradict itself, therefore, it is up to us by study and Spirit to allow time for Scripture to show its' own solution.
1. It is clear that not being under the law but under grace does not mean we are without law: lawless. Riotous living (Luke 15:13)(1 Peter 4:4). Criminal behavior. (1 Peter 4:15)
2. It is also clear that we are under law to Christ and not without law to God.
Therefore, when Scripture speaks of Christians not being under law, and yet being under law of Christ and not without law to God, then plainly we are not under any law, that is not to God and of Christ.
And since the context of a law that is not of Christ, is to the Jews, then Scripture must be referring to the law of Moses, which the Jews kept to that day:
For Moses of old time hath in every city them that preach him, being read in the synagogues every sabbath day. (Acts 15)
So, we are not under law of Moses, and thus are not judged thereby: No man can be called transgressor by God, if any man transgresses law of Moses. However, we are with law to God, and we are under law of Christ: Any man can be called transgressor by God, it any man transgresses the law of Christ.
Where no law is, there is no transgression (Rom 4:15). And since there is law of Christ to God, then there is transgression against it, by any person.
The error therefore is twofold: not taking all Scripture into account, and so not taking necessary context into account:
"Christians are not under law at all" is an obvious lie against Scripture, which would then mean Christians are without law to God: having no law of God to do and to keep, lest we also become transgressors of God's law of Christ:
For if I build again the things which I destroyed, I make myself a transgressor. (Gal 2)
For he that said, Do not commit adultery, said also, Do not kill. Now if thou commit no adultery, yet if thou kill, thou art become a transgressor of the law. (James 2)
Not being under law other than that of Christ, and being under grace, is not to be without law, and lawless, just because we are saved by grace.