"Anyone who runs ahead and does not continue in the teaching of Christ does not have God; whoever continues in the teaching has both the Father and the Son." NIV translation
When he says "runs ahead", he is talking about going further than is right. At this time, there were all kinds of people going around claiming to have a super knowledge of God. The deceivers had gone among them and claimed to have a deeper knowledge of God and that did not include Jesus as the Son of God. John was letting them know that, without the Son, they didn't have the Father either. Today, there are many who consider themselves to be "super saints" or holy rollers" because of some obscure bit of knowledge that they think they have. They get all caught up in what they think they know or their own actions and leave Jesus out. But, as John told them here, without Jesus at the center, we have no relationship with God the Father.
"If anyone comes to you and does not bring this teaching, do not take him into your house or welcome him. Anyone who welcomes him shares in his wicked work." NIV translation
Here we have the "apostle of love" telling this woman not to welcome (and show them love) these deceivers. He even warns her that to do so is to be a part of their evil work.
This passage has been used to cause much division among believers but it was not intended to be that way. John gives a specific reason for not welcoming these people and that is the fact that they deny Jesus as being the Son of God. There are many people in the world today that are trying to get Muslims and Christians to join together for the common good in what has been commonly called "Chrislam". John is warning against that type of thing.
"I have much to write to you, but I do not want to use paper and ink. Instead, I hope to visit you and talk with you face to face, so that our joy may be complete."" NIV translation
In our world of cell phones and all of the gadgets, it may be hard for us to understand but John is telling them that he would prefer to talk to them face to face. That is how you have fellowship and joy in coming together around the things of Christ.
"The children of your chosen sister send their greetings." NIV translation
No one is quite sure about who this verse refers to. It could be a sister or a sister church.