A story I have heard in church... A long long time ago for me lol.
Missionaries would go to Africa, and bring the gospel to the people. When the people started to believe, Jesus became just one of their many Gods, but they worshipped Him in truth, but he was just one of their Gods.
When one of the missionaries was telling the people off for worshipping other idols an "divine" thought struck him "Who are you to condemn these people, don't you do just the same with money, tv, sex, etc..."
It became clear to him, that the mordern man had many idols, they just weren't seen as idols. Who sees the tv as an idol, that people worship? noone right? who sees sex as an idol that people worship? noone right? but yet they take up the same place in the row besides Jesus, making Jesus just one of the Gods they have.
Don't forget when talking to people that believe in multiple Gods that they have different viewpoints. The thing to do is to learn why they think what they think. And use that information in friendly dialogue to try to disprove them.
Michael