At some point during the conversation after they realize their points have no validity or you have asked them a question they know they can't answer honestly without admitting their wrong, you are almost assured that they will bring up how you follow the Nicene creed. Rather than rehash the same old argument I encourage you to read Eric Johnson's posting on the MRM Blog.
Because the LDS feel they have to work in order to make themselves worthy before God they in turn see the Bibles grace as cheap. For if a person claims to be forgiven before they earn it then they can go around killing, stealing, and lying with no worries of punishment for the sin. You will often see Mormons use the second half of Philippians 2:12 out of context to show how we need to work for salvation however lets look at it shall we.
"Wherefore, my beloved, as ye have always obeyed, not as in my presence only, but now much more in my absence, work out your own salvation with fear and trembling."As we can see from the first portion of the verse he is addressing those who are already saved, not those who are working towards it. As a result of being saved we then go out for works knowing the price that was paid for them. (Ephesians 2:10)
As a result of a Mormons upbringing they assume that the church is the building a person attends and defines their version of truth. They think that the church we attend defines our definition of truth like it does theirs. Any Born Again Christian understands knows that the building someone choices to worship in doesn't always define their beliefs. A true Christian rely's on the Word of God to mold their will to His, not a self proclaimed prophet. Jesus is our Truth (John 14:6) not a church.