The central theme of the gospel of Mark concerns the identity of Jesus. This is why one the great questions of the gospel can be found in the words of Jesus, "Who do men say that I am?" In light of this, the gospel of Mark shows that Jesus is incessantly misunderstood. So, when we come to Mark 2 when Jesus's ministry is in full swing, people are confused by him and there is religious tension.
So, when Jesus heals a paralytic man and even says that his sins are forgiven, people are in an uproar, because they misunderstand him. When he eats with Levi, a tax collector, there is also tension, as tax collectors were the most hated people in society. People do not realize that Jesus came to save "sinners" like the tax collector. Or to use the words of Mark, "the Son of Man has come to seek and save the lost." Likewise, there is further misunderstanding when he reinterprets the intention of the Sabbath.
So, the religious truth that is communicated in Mark 2 is that Jesus is God. He is the long awaited messiah. This is why he can forgive sin, save sinners, and reinterpret the law.