Many well-known figures in the Seventh-Day Adventist Church (such as Doug Batchelor) have, for decades, made the claim that what Christian’s believe about the soul continuing after the physical body dies to not be derived from scripture, but mythological teaching from the Dark Ages.
This is not true. The Adventist Church is notorious for rewriting history through the lens of their Great Controversy paradigm.
We have multiple, early christian source documents that show Christians from the time of the Apostles all believed that the soul continues after one dies and they pointed to scripture and the Apostles for their support. This was hundreds of years before even the infancy of the Dark Ages. You can see a few of these sources here.
The more honest representation that the Adventist Church (and Adventist’s) should give is that they disagree with the interpretation of the early Christians. Not lie and say that the early church actually believed like Adventist’s do or that this belief is an extra-biblical concept that the early church got from somewhere outside of the Bible.
The early Church pointed to things such as Matthew 10:28 where Jesus made a dichotomy between the body and the soul when He said to not fear those that can only kill the body, but fear Him who can cast both soul and body into Hell. If the Adventist view were correct (that the soul is a combination of breath and a body), then it would be possible for man to kill the soul by killing the body. This is just one of the many places the early church pointed to support their view.
On the one hand the Adventist Church will tell you they are a restoration of ancient Christianity with all of the extra-biblical stuff that crept in during the Medieval period removed. But then when it comes to examining what the early Church looked like, they dismiss them as not believing the Bible, appealing to extra-biblical sources for their belief, etc. Quite the conundrum.