The theological ideas are ultimately harmonious. We should be neither anti-LXX nor anti-MT.
I post to counteract the idea the apostles *only* worked from a standard Greek LXX, or that the MT has been systematically corrupted.
On the contrary, it’s sometimes *more* messianic.
Isaiah 9:6 is also more messianic in the DSS & MT than the majority LXX.
Of course, the apostles usually quote the LXX, and the LXX & MT are normally very close.
Occasionally, the apostles quote the LXX divergently from the (proto-)MT—like Hebrews 10’s use of Psalm 40 (39 LXX).
Ephesians 4:30:
“do not grieve [lypeō] the Holy Spirit of God…”
Isaiah 63:10 (MT):
“they grieved His Holy Spirit…”
Isaiah 63:10 (LXX):
“they resisted and provoked [parōxynan] His Holy Spirit…”
Romans 16:20:
“the God of peace will *crush* Satan under your feet.”
Genesis 3:15 (MT):
“her seed will *smite* you on the head…”
Genesis 3:15 (LXX):
“her seed will *carefully watch* your head…”
Romans 1:5:
“for the obedience of faith among all the nations on behalf of his name…”
Genesis 49:10 (MT):
“And to him shall be the obedience of nations.”
Genesis 49:10 (LXX):
“he is the expectation of nations.”