"You hypocrites! Isaiah was right when he prophesied about you: 'These people honor me with their lips, but their hearts are far from me. They worship me in vain; their teachings are but rules taught by men.'" Matthew 15:7-9 The New International Rainbow Study Bible
When we talk about the Ten Commandments and realize we are told not to take the Lord's name in vain, we are taught by men that it means not to say "oh my god" or similar little sayings. I don't think that's what is meant by taking His name in vain and this scripture is one of the reasons.
According to Jesus, worshiping in vain has to do with saying religious things when you don't believe or feel them. It means simply spouting the words we have heard from men, not from Jesus or God.
I once had a group of friends, members of a book club. We were discussing parts of the New Testament one night. Several of the members spoke with great authority about the scriptures they had studied in various Christian denomination churches. During the evening it became apparent to all of us that they did not believe in the virgin birth or that Jesus rose from the grave. This, I believe, is what Jesus meant when he said people worship him in vain and what God meant in his commandment.
Being a Christian does not mean mouthing the words we read or hear from sermons. It means really believing the unbelievable -- what is hard to swallow.
We are not taught it is easy to be Christian. We are told it is very hard. People look to believers for the truth.
If we don't really believe with our hearts and minds, we are just talking when we speak of God, the Father, and Jesus.