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.http://www.gotquestions.org/Seventh-Day-Adventism.html
“Seventh-day Adventists (SDAs) follow most of the beliefs of conventional conservative Christianity: creation in six days, the fall of mankind in the Garden of Eden, original sin; the virgin birth; the divinity of Christ; the nature of the Trinity; belief in Satan as a rebellious created being; God’s inspiration of the authors of the Bible, the inerrancy of Scriptures as they were written by their authors; the resurrection of Jesus, salvation by the atonement of Christ, considering all same-gender sexual behavior as sinful, regardless of the nature of the relationship; rejection of same-sex marriage, civil unions, etc.” http://www.religioustolerance.org/sda2.htm
They deny the concept of “innate immortality“. They believe that a person is not naturally immortal. When a person dies, they remain unconscious until they are resurrected. Eternal life is a gift which God gives only to righteous Christians; the rest will be ultimately annihilated and no longer exist in any form. Thus, they do not believe that a person goes to heaven for an eternal reward or to hell for never-ending torture immediately upon death as do many other conservative Christian denominations.
The Seventh-day Adventist church takes a position between the strict pro-life and strict pro-choice alternatives. A set of “Guidelines on Abortion” was approved by theGeneral Conference of Seventh-day Adventists Executive Committee on 1992-OCT-12. It says in part:
“The Church does not serve as conscience for individuals; however, it should provide moral guidance. Abortions for reasons of birth control, gender selection, or convenience are not condoned by the Church. Women, at times however, may face exceptional circumstances that present serious moral or medical dilemmas, such as significant threats to the pregnant woman’s life, serious jeopardy to her health, severe congenital defects carefully diagnosed in the fetus, and pregnancy resulting from rape or incest. The final decision whether to terminate the pregnancy or not should be made by the pregnant woman after appropriate consultation. She should be aided in her decision by accurate information, biblical principles, and the guidance of the Holy Spirit. Moreover, these decisions are best made within the context of healthy family relationships.”
The church follows the practice of the earliest Christian movement: the Jewish Christians who were located in Jerusalem under the leadership of James, a brother of Jesus. They observed the Jewish Saturday Sabbath because of very clear instructions from God that were to stay in effect forever. One example is Genesis 2:2-3. it describes how God rested on the seventh day, Saturday, after having spent the previous six days creating the world, its life forms and the rest of universe. God is recorded as blessing the day and making it holy. It was apparently created as a day of rest for all mankind, forever.
The Seventh-day Adventist church traces its roots to American preacher William Miller (1782–1849), a Baptist who predicted the Second Coming would occur between March 21, 1843, and March 21, 1844. Because he and his followers proclaimed Christ’s imminent advent, they were known as “Adventists.”
When Christ failed to appear, Miller reluctantly endorsed the position of a group of his followers known as the “seventh-month movement,” who claimed Christ would return on October 22, 1844 (in the seventh month of the Jewish calendar).
Seventh-day Adventists agree with many Catholic doctrines, including the Trinity, Christ’s divinity, the virgin birth, the atonement, a physical resurrection of the dead, and Christ’s Second Coming. They use a valid form of baptism. They believe in original sin and reject the Evangelical teaching that one can never lose one’s salvation no matter what one does (i.e., they correctlyreject “once saved, always saved”).
Unfortunately, they also hold many false and strange doctrines. Among these are the following: (a) the Catholic Church is the Whore of Babylon; (b) the pope is the Antichrist; (c) in the last days, Sunday worship will be “the mark of the beast”; (d) there is a future millennium in which the devil will roam the earth while Christians are with Christ in heaven; (e) the soul sleeps between death and resurrection; and (f) on the last day, after a limited period of punishment in hell, the wicked will be annihilated and cease to exist rather than be eternally damned. (For rebuttals of many of these ideas, see the Catholic Answers tracts, The Antichrist, The Hell There Is, Hunting the Whore of Babylon, The Whore of Babylon, and Sabbath or Sunday?)
Many Adventists insist that, as a matter of discipline (not doctrine), one must not eat meats considered unclean under the Mosaic Law (many endorse total vegetarianism), and one must avoid “worldly entertainments” (card-playing, dancing, smoking, drinking, reading non-religious books, listening to non-religious music, watching non-religious television, going to the movies, etc.). Read more at: http://www.catholic.com/tracts/seventh-day-adventism