A question was posed to Bro Abdul Rahman during the question and answer session (Daawah Technique lecture) :

What if we don't have enough knowledge? How can we do daawah?

correct me if i'm wrong. Answer, if you do have the knowledge, if you don't, say I don't know but I'll get back to you. Then you search for it. No one knows everything.

The lecture can be watched online here, credit to anas for recording and nazirah for informing =)