Mostly, I think my ability to evaluate this argument is distorted beyond reliability by including the word "Christianity." So, first, let me try and taboo that word and generate a more generalized version of the argument:

1/2. My values are primarily learned from society, not innate.

1. There exists some tradition from which society's values were primarily derived.
   4/5. My values, once learned, can be later modified. Most such modifications are harmful from the perspective of those values.
2. Those (harmful) modifications can be countered by reverting to a reliable specification of the tradition from which society's values were primarily derived.
3. Therefore doing so is beneficial.

That's far from airtight, but it's not altogether unreasonable.

My major problem with it is that, once my values drift, there's no obvious reason why I should prefer my original values... so it's really more of an argument for continuing to follow that specification rather than for choosing to in the first place. But admittedly moral drift in the real world is usually not an all-or-nothing thing (that is, I can continue to mostly hold onto my core values even while I'm in the process of drifting away from them), so that's not a debilitating problem.

So yeah, I would accept a weak probabilistic form of the conclusion: all else being equal, it's usually beneficial for me to follow a reliable specification of yadda yadda.

And, sure, it follows from that that if there exists something called Christianity which is a reliable specification of yadda yadda, all else being equal, it's usually beneficial for me to follow Christianity.

That said, I've never seen anything that matches that description in the real world.