If a lecture is given by a scholar who is wellknown for sound creed and vast knowledge, you will have done a good job to give it to Muslims as a gift and will have a reward similar to that of them as the Prophet (peace be upon him) said: One who guides to something good has a reward similar to that of its doer. (Narrated by Imam Muslim in his Sahih).
At the same time, there is no harm if you speak about what you know of truth with a good style. For example, you can urge people to offer prayer in congregation, to pay Zakah, and warn them against backbiting, slandering, disobeying parents, severing ties of relationship and abstaining from committing the Fawahish (Great sins) that Allah (Exalted be He) has forbidden because these things are known to all Muslims.