Christians are to be different than the world around us. We have a different set of values, beliefs, and priorities than those who are not Christians; it's what marks us as different from everybody else. How do you know somebody is a Christian? Well you should be able to tell they're different by their actions [Galatians 5.22-26]. It should be evident in every level of their life-we're the ones who live with the Holy Spirit guiding our lives.
This mean that even the very WAY that we speak - our rhetoric - should be DIFFERENT. The very way that we communicate to people around us should be influenced by the Spirit of God that is present inside our hearts. It is not ok for a Christian to talk like everybody else.
Now what does it mean for a Christian to speak like a Christian? Well, I think a great place to start is the above mentioned passage of Scripture -- Galatians 5.22-26, the Fruit of the Spirit. The Fruit of the Spirit are: love, joy, peace, patience, kindness, goodness, faithfulness, gentleness and self-control. So a Christian, when they speak, should be reflecting these attributes; these fruit of the Spirit.
The question we should be asking ourselves: "If a Christian, when they speak, is not revealing the Fruit of the Spirit, is the Spirit in them?" or perhaps we should ask,"If a Christian, when speaking, is not showing these Fruit of the Spirit, are they speaking like a Christian?" Tough questions, but I think important ones to ask ourselves...
I have seen too many Christians who have taken the Gospel message - a message of LOVE & HOPE - and not present a message that is neither of loving nor hopeful. If the rhetoric we use to communicate the Gospel isn't loving, then perhaps we're not presenting the Gospel message!
If we're not speaking without showing the Fruit of the Spirit, then perhaps we need the Fruit of the Spirit.