When is a Muslim obligated to engage in da'wah (inviting to Islam), and is it after having knowledge of what he is inviting to?

Calling to Allah is obligatory upon every Muslim, due to the Prophet's (peace be upon him) saying: "Convey from me, even if it is a single verse." This call must be based on knowledge and insight. However, this does not mean that the caller must be knowledgeable in all matters of religion; rather, it suffices for him to know one issue from its issues to convey it to Muslims or to those who need it.