A fine first firearm and great question to boot. A large black bear can tip the scale at 550 pounds, and the heaviest wild American black bear ever recorded weighed approximately 1,100 pounds! That’s a monster, and that’s why you know deep penetration is crucial.
Now, in all honesty, 125 grains is relatively lightweight for a 357 Mag bullet. That means it may not possess the mass and resultant momentum requisite to penetrate to an exceptionally hefty bear’s vital organs. For that matter, an expanding bullet like the FTX may not penetrate deeply enough, either. For defense against bears with a handgun, the consensus is generally that heavy, non-expanding bullets are best. A hard cast bullet in particular is able to resist the kind of deformation that could limit its potential for deep penetration.
We would personally prefer a 180 grain hard cast if we were trekking through Yogi territory. If you’re concerned about recoil and you don’t intend to encounter any unusually large black bears, a 125 grain FTX should prove enough to discourage a curious bear from sharing your oxygen. Just “bear” in mind that a large and enraged bear may prove too much for a lightweight expanding projectile to deter.