In answering this question I’ll outline quickly some of the Seventh Day Adventists beliefs, but because of space and because I’m not an expert on this, this will only be brief. For more information, check out the SDA website or a good dictionary.

SDA beliefs can be summarised into three main points:

  1. They believe that the Bible predicts 1844 to be the time when Jesus begins an"investigative judgement” in the heavenly realm
  2. They hold strongly that Christians should worship on the Sabbath, namely Saturday
  3. Most SDAs generally hold Ellen White as a prophet for the end time church. They generally hold her writings as authoritative, possibly on the same level as the Bible itself, as the inspired word of God.

The concept of"Investigative Judgement” refers to the idea that the confessed sins of believers are forgiven but not blotted out until an"investigative judgement” occurs and the ruling declared favourable. This occurs after someone comes to faith, but before the second coming. The blotting out of sins occurs by sins being placed on Satan as the scapegoat.

The most significant error in the doctrine of the SDA probably lies in its denial of the all sufficiency of Christ’s atonement for the removal of sins. There is no assurance of salvation for the believer. The idea of investigative judgement is based on a faulty interpretation of Daniel 8:14 and a mistaken understanding of the Old Testament sacrificial system.

Romans 3:21 - 25
But now a righteousness from God, apart from law, has been made known, to which the Law and the Prophets testify. This righteousness from God comes through faith in Jesus Christ to all who believe. There is no difference, for all have sinned and fall short of the glory of God, and are justified freely by his grace through the redemption that came by Christ Jesus.

Romans 4:23-25
The words “it was credited to him” were written not for him alone, but also for us, to whom God will credit righteousness””for us who believe in him who raised Jesus our Lord from the dead. He was delivered over to death for our sins and was raised to life for our justification.

The Bible clearly shows us that believers are"freely justified” by grace and the righteousness of Jesus is credited to believers at the time they believe.

Colossians 2:16-17
Therefore do not let anyone judge you by what you eat or drink, or with regard to a religious festival, a New Moon celebration or a Sabbath day. These are a shadow of the things that were to come; the reality, however, is found in Christ.

A second major difference is the legalistic tendency in the SDA religion, in their beliefs that they should only worship on Saturdays and that to be part of the church they must abstain from alcohol, tobacco and certain food forbidden by the Old Testament laws. The Colossian passage above demonstrates that no day is more holy than another and we should not be judged by what we eat or drink.

In approaching someone from a SDA background, it is important to be loving and gentle always. The best approach would be to acknowledge their recognition of the Bible as the Word of God and read the Bible with them, especially looking at passages (e.g. Romans 1-5, Galatians) that clearly explains justification by grace alone, not works, and the full assurance of salvation Christians can enjoy once they believe. It may also be helpful to point out that the Bible is the final authorative word of God and everything, including prophecies, must be measured up against it.

(Information in this answer drawn largely from Encyclopedic Dictionary of Cults, Sects and World Religion, by Nichols, Mather, Schmidt, published by Zondervan 2006)