There are many differences between Catholics and Seventh-day Adventists, as there are between Catholics and most Protestants.
Some of the differences between them are:
Catholics have a priesthood while Adventists support the priesthood of all believers.
Adventists do not recognize the Pope, the President of the Adventist church has no doctrinal authority.
Adventists worship on Saturday and Catholics worship on Sunday.
Catholics believe that Mary was born perfect (without original sin), remained a virgin her whole life, was taken into heaven at the end, and serves as coredemptrix with Christ. Adventists believe that Mary was a simple, imperfect girl; that she was a virgin until after Christ's birth; that she died, was buried, and is still in the grave; and that Jesus is the only Redeemer. Adventists believe that people rest in the grave when they die (soul sleep) until the Second Coming of Jesus, with very few exceptions (e.g. Enoch, Elijah, etc.).
Catholics accept the adoration of images and saints; Adventists do not.
Adventists practice Communion (similar to the Eucharist) and believe in consubstantiation (the bread and Grape Juice [not wine] represent the body and blood of Christ); Catholics practice the Eucharist and believe in transubstantiation (that the bread and wine become the actual body and blood of Christ).
Another answer:
Adventist are perhaps more similar to Catholics that most Protestants, Adventists recognize an extra-Biblical source and some Adventists deny righteousness by faith alone.
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