People hate preachy songs. So if you're hellbent on writing one, it had better be really really good. The Times They Are A Changing had a memorable tune and fairly poetic lyrics and was addressing people other than those who were listening (thereby rendering it 'subtle' instead of 'let me lecture you, you oaf') That approach and quality were the only reason people tolerated it.   

Anyway, if you still want to deliver such a message, either send it Western Union as the old joke goes, or sit down and write a human story that gets your point across without the sermon.