What does Winston say is the one thing he and Julia must never do?
To answer this question, take a look at this quote from Winston in part two, chapter seven, when Winston and Julia are discussing the concept of betrayal:
Confession is not betrayal. What you say or do doesn’t matter: only feelings matter. If they could make me stop loving you—that would be the real betrayal.
In other words, Winston and Julia agree that they must not let the Party get into their heads and manipulate their thoughts because they know that if this happens, their love for each will be destroyed. That is, in their opinion, the ultimate betrayal.
For Winston and Julia, it is better to confess to their crimes, like joining the Brotherhood, than it is to stop loving each other. After all, they know that if they are captured and tortured, they are likely to confess to their crimes, anyway.
For them, the most important thing is to not stop loving each other. If the Party is able to take that away, then all hope is lost.