When you see people offering free English lessons be careful to find out what their real motivation is. Some people I know for a fact are here to teach English in order to convert "infidels" to Christianity. I have had these people try to convert me! Others are offering free lessons because they have a political or ideological agenda to promote and others might have other reasons that you should be aware of.  

I'm not saying that there aren't people in the world with generous and kind hearts that are only wishing you well, I'm just suggesting that you make sure what the real motivation is by ASKING them what it is you will be talking about. The people that are here only to spread the word of their religion will usually say that they want to use some scripture to read to help you learn English. They will usually target people from countries that are not Christian usually. Others might want to promote the opposite, only a very scientific way of looking at the world to convert you away from your religions. Others will tell you that they want to talk about politics.

There are few people in the world that give freely, expecting absolutely nothing in return, so just be sure that you know for sure that the person you are trusting to teach you, really is this kind of person. Look at their posting history, how others respond to them here, if they tend to talk about only one subject and what topics they tend to contribute to. 

If you are willing to listen to someone lecture you on religion, science or politics in order to learn English, then you need not worry at all. :D If however you are sensitive to these things and do not know that people here who are offering free English, but it's actually not free, it's to promote their cause, now you won't be surprised. 

Again, ask questions, look at their histories and take care! 

Namaste