Is it un-Christian to criticize Mormonism?

I posted a response to this comment posted on Mormon Coffee.

Andrea replied:
you can't be serious. The Latter-Day Saint Church is all about coming to Christ. It's part of being a Mormon. I have a strong testimony of Jesus Christ and serving Him. I am doing a research projoect for school and when I come across these "chip-on-the-shoulder" ex-mormons, I can't help but wonder why they have such negative feelings for other Christians if they claim to follow Christ themselves. I grew up in the church and was inactive for many years. Do you know what I found out there in the world? Here a little and there a little. It took a few years of being stupid to realize I was wasting my time away from God and His TRUE CHURCH. You can pick it apart if you want to, but is that really helping you toward salvation? The answer is no. May the spirit touch your heart and bear witness as it does, that Joseph Smith was a prophet. Have you read the articles of faith? It talks about believing the scriptures as far as they are translated correctly. So, pray and ask if it's true.

My response:
I know that many Mormons sincerely seek to heed the admonition to come to Christ (2 Nephi 25:26) including my own children who are still LDS. However, Mormonism--by its own definition--teaches a different Christ than the traditional church (JSH 1:19). That distinctiveness was reiterated by President Hinckley in 1998 (click for Church News article).

The Bible teaches that a correct knowledge of the nature of God is paramount to salvation (John 17:3) and Joseph Smith agrees: "It is the first principle of the gospel to know for a certainty the character of God" (History of the Church, 6:305).

I appreciate your testimony and hope that I am never guilty of criticizing LDS people for their sincerity nor the experiences which lead them to their faith. But I must put everything to the test (1 Jn 4;1; 1 Thes 5:21) for the sake of my own salvation and preach the true gospel for the salvation of others (Romans 10:10).