"Christians only worship God."
To be precise, Christians worship Christ. Christ is best considered a representation of God, but not all of God.
Muslims find this confusing, as they feel God is the more important thing and Christ only a representative (one of the prophets of God, like Muhammed, who came after Christ and is therefore "more accurate"). Conversely, Christians believe that Christ's life constitutes the miracle which connects mankind fo God, and is therefore the single essential thing to believe in (God is therefore implicit in the belief in Christ).
Some think Catholics also worship the Virgin Mary, but in fact this is not the case. The Virgin Mary is not believed to have any divinity herself, but is a conduit to the divine power of God or Christ, being in a privileged position. Also seen as an advocate for womens' issues.