بسم الله الرحمان الرحيم
وَقَدْ نَزَّلَ عَلَيْكُمْ فِي الْكِتَابِ أَنْ إِذَا سَمِعْتُمْ آيَاتِ اللَّهِ يُكْفَرُ بِهَا وَيُسْتَهْزَأُ بِهَا فَلَا تَقْعُدُوا مَعَهُمْ حَتَّى يَخُوضُوا فِي حَدِيثٍ غَيْرِهِ إِنَّكُمْ إِذًا مِثْلُهُمْ إِنَّ اللَّهَ جَامِعُ الْمُنَافِقِينَ وَالْكَافِرِينَ فِي جَهَنَّمَ جَمِيعًا
"And it has already been revealed to you in the Kitaab that when you hear the Aayaat of Allaah being rejected and mocked at, then do not sit with them (those who are doing so) until they engage in some other discussion; otherwise, you will be like them (i.e. one of them). Indeed, Allaah will gather the Munaafiqeen and the Kaafireen together in Jahannam." [Soorah an-Nisaa', 4:140]
A very grave warning has been sounded in this Aayah. The `Ulamaa explain that when people are uttering Kufr and/or mocking Islaam, you have only one of two options:
- Stop them
- Leave
They explain that if a person remains sitting in a gathering where Islaam is being mocked, without stopping those who are doing so (physically) or leaving, then this person has committed Kufr, wal-`Iyaadhu Billaah. May Allaah Ta`aalaa protect us from that, Aameen.
So if these people - as you say - are simply mocking Islaam and committing Kufr in this manner and have no intention of learning, and thus you telling them about Islaam will have no benefit, then it is not permissible for you to sit with them or be part of their gathering. Instead, you should sit with Muslims and engage in discussions wherein there is benefit for you in the Dunyaa and the Aakhirah.
Look at the Words of Allaah Ta`aalaa in this Aayah:
"Indeed, We will gather the Munaafiqeen and the Kaafireen together in Jahannam."
Allaah Rabbul `Izzat mentions the Munaafiqeen here even before the Kuffaar. From this, the `Ulamaa have derived that the person who sits in gatherings wherein Islaam is mocked without stopping them or leaving the gathering has become a Munaafiq.
You did the right thing by leaving, akhi. Do not be on such forums. You have to either refute them and disassociate yourself from them, or leave the forum.
I don't suggest you trying to refute them, though. Leave it to those who are skilled in the field of debate and comparative religion. If a person does not have enough knowledge and tries to debate, it could end up putting doubts in his own mind. So best you stay away from those forums.