The Seventh Day Adventists are best known for their promotion of "Seventh Day" (Saturday/Sabbath) Worship (which I've discussed Here), but surprisingly that Sabbath teaching isn't even their most important doctrine! Since SDAs are quite often militant against Catholics, I think this apologetics piece will be quite handy in stopping them dead in their tracks. The key is to look at the other half of their name: Adventist.
The SDAs have "28 Fundamental Beliefs" [Here] which are basically their unique set of dogmas that are required believing for any SDA to be in good standing. While everyone is aware of their Sabbath teaching, what is far less known is that the SDAs have a dogma concerning the year 1844, and the reason why you don't know about it is because it is quite embarrassing for the SDAs. The full text of the Fundamental Belief #24 says:
There is a sanctuary in heaven, the true tabernacle that the Lord set up and not humans. In it Christ ministers on our behalf, making available to believers the benefits of His atoning sacrifice offered once for all on the cross. At His ascension, He was inaugurated as our great High Priest and, began His intercessory ministry, which was typified by the work of the high priest in the holy place of the earthly sanctuary. In 1844, at the end of the prophetic period of 2300 days, He entered the second and last phase of His atoning ministry, which was typified by the work of the high priest in the most holy place of the earthly sanctuary. It is a work of investigative judgment which is part of the ultimate disposition of all sin, typified by the cleansing of the ancient Hebrew sanctuary on the Day of Atonement. In that typical service the sanctuary was cleansed with the blood of animal sacrifices, but the heavenly things are purified with the perfect sacrifice of the blood of Jesus. The investigative judgment reveals to heavenly intelligences who among the dead are asleep in Christ and therefore, in Him, are deemed worthy to have part in the first resurrection. It also makes manifest who among the living are abiding in Christ, keeping the commandments of God and the faith of Jesus, and in Him, therefore, are ready for translation into His everlasting kingdom. This judgment vindicates the justice of God in saving those who believe in Jesus. It declares that those who have remained loyal to God shall receive the kingdom. The completion of this ministry of Christ will mark the close of human probation before the Second Advent. (Lev. 16; Num. 14:34; Ezek. 4:6; Dan. 7:9-27; 8:13, 14; 9:24-27; Heb. 1:3; 2:16, 17; 4:14-16; 8:1-5; 9:11-28; 10:19-22; Rev. 8:3-5; 11:19; 14:6, 7; 20:12; 14:12; 22:11, 12.)
You might be having a hard time understanding what is being said in the above FD#24 about this "Investigative Judgment" (I.J.), but that's because it isn't meant to make sense. The dogma is complete nonsense, reworded in order to save face as to the original meaning. If you read the official page dedicated to explaining this IJ [here] you will see that it spends many paragraphs saying nonsense. However, it does give us some details as to what it all originally meant, such as the following quote (trimmed back for length):
