You don’t accuse somebody of lying just because they said something that is false. The accusation of lying is only justified if you have good evidence that they knew that what they were saying is false at the time that they said it and that they said it with the intention of persuading someone else that it is true.
Something similar may be said with respect to the fallacy of strawmanning. It is not sufficient to argue that a person has presented an opposing view inadequately. One must have evidence that they were aware of superior presentations of the view at the time that they shared the inferior one and that they shared the inferior one with the intent of misrepresenting the strength of the view.
As Jacob Van Vleet puts it, strawmanning “is an argument that intentionally presents a misrepresentation of a particular position in order to easily refute or dismiss it.” Van Vleet gives this example: “Jews and Christians simply believe that if you are good you will go to heaven, and if you are evil you will go to hell. This is the gist of their belief. So, you shouldn’t take them seriously.” As Van Vleet points out, this description of what Christians believe is simply not accurate: “In this example the views of Jews and Christians are intentionally distorted so that they can be easily dismissed.”
It follows that if a person really doesn’t understand Christianity such that they really do think that heaven is merely a reward for good behaviour, then they are guilty of ignorance but not of strawmanning. So before you accuse somebody of strawmanning, be sure that you have sufficient evidence to establish intention.