Question: "What is Seventh-day Adventism (SDA), and what do Seventh-day Adventists believe?"
Answer: There seem to be different "degrees" of Seventh-day Adventism. Some Seventh-day Adventists believe identically to orthodox Christians, other than believing that worship should be held on Saturday and that the Saturday Sabbath should still be observed. If these are the only differences, then, yes, a person could be a Seventh-day Adventist and still be a true believer in the Lord Jesus Christ.
However, some Seventh-day Adventists believe in much more than a Saturday Sabbath / worship day. Seventh-day Adventists have been known to believe in the annihilation of the wicked instead of an eternal hell, and believe that believers who die enter a state of soul sleep and that a person must observe the Saturday Sabbath in order to be saved. Other problems with some Seventh-day Adventists are belief in Ellen G. White, the founder of Seventh-day Adventism, as a true prophet of God, even though many of her "prophecies" failed to come true, and that Jesus entered a second phase of His redemptive work on October 22, 1844, as "prophesied" by Hiram Edson.
So, what is Seventh-day Adventism, and what do Seventh-day Adventists believe? Should a Christian attend a Seventh-day Adventist church? Due to the potential doctrinal issues mentioned above, we would strongly encourage believers to not get involved in Seventh-day Adventism. Yes, a person can be an advocate of Seventh-day Adventism and still be a believer. At the same time, there are enough potential issues to make attending a Seventh-day Adventist church questionable at best.