"If a new body can be created to house it, a soul can be resurrected. If a human soul can be preserved repeatedly, then it can live forever." Or so the man claimed.
The reality of the situation is that a homunculus can only live within the container it was created in: if it leaves, the body collapses and it dies instantly. The church excommunicated the man, burned his research, and explicitly forbade any further research on homunculi.