Now in short, concerning the actual lecture, I left the hall 2 hours later feeling like I'd been told off. Like I'd just had a good scolding that was well deserved! As Muslims we have the responsibility of inviting non-Muslims into Islam. It is obligatory. All of us and the least we can do as he put it is to 'Open your mouth.' At least open your mouth when others speak to you of their religion, of their 'Gods'. We would be asked of this on the day of judgment. No supreme knowledge is needed, nor are you being asked to fight other religions. All he's asking of us is to at least open our mouths. Just ask them 'Why do you believe he's your God?' Take it from there...
If you're being asked to mind your own business, then inform them that you are. It is a Muslim's business to pass on the message that there is no God but Allah(swt) and Muhammad (peace be upon him) is his messenger.
Allah (subhanahu wa Ta`ala) says in the Quran:
"Invite to the Way of your Lord (i.e. Islam) with wisdom and fair preaching, and argue with them in a way that is better. Truly, your Lord knows best who has gone astray from His Path, and He is the Best Aware of those who are guided." (Quran, an-Nahl: 125)
"Let there arise out of you a group of people inviting to all that is good, enjoining al-ma`roof (i.e. Islamic Monotheism and all that Islam orders one to do) and forbidding al-Munkar (polytheism and disbelief and all that Islam has forbidden). And it is they who are successful." (Quran, Aal Imraan: 104)
That night after we left, we unfortunately missed the question/answer session where people converted into Islam.
So if you're a non Muslim reading this, please look into Islam with an open mind and a clear heart. As for my Muslim readers, we better start opening our mouths...
The Prophet (p.b.u.h.) has said: "Convey from me, even one verse." (Bukhari)