"idam te natapaskaya
na chasusrusave vacyam
"Sri Krishna said: O Arjuna, This confidential divine knowledge should never be explained to those who are not pure, austere, or are not devoted to God, or are not engaged in devotional service, nor to one who is envious of God."
"ya idam paramam guhyam
bhaktim mayi param kritva
"Sri Krishna said: O Arjuna, one who explains this supreme divine knowledge secret to the devotees of God, pure devotional service is guaranteed to that person, and at the end he will surely achieve the God."
So we should not unnecessarily teach & preach the people of bad nature who hate God. Such people cannot be changed of their bad nature as they are engrossed always in bad karmas & are always envious of God. We should let such people live in their own world. But if we shall impart divine knowledge to the people who are devoted to God then it shall be the real beneficial service to God. In this way God shall endow us with devotional energy through which we shall finally attain Him.