Leaking threaded pipe joint

[brimanley]

Updating an old home bathroom by putting in new tub/shower. I had to cut the metal pipe to make the new drain and trap work, but the pipe turned where it joins up to the toilet drain. This caused a small drip leak so I cut another hole by the toilet and cut off the pipe about 8inches from the bell but could not unscrew it.

I really don't want to get into replacing all the cast iron plumbing into the main stack. What is the best way to reseal the threaded joint? The pictures show the drain pipe going into the toilet drain.

Thanks