It's an interesting one, and I don't really want to get into the relative weight of the evidence, but
¶ 2,1 ⸀ μαρτυριον ℵ2 B D F G L P Ψ 33. 81. 104. 365. 630. 1175. 1241. 1505. 1506. 1739. 1881. 2464 M b vg syh sa ╎ txt P46vid ℵ✱ A C ar r syp bo; Hipp BasA Ambst
The NA28 apparatus shows the issue. The evidence for μαρτυριον is quite wide and multi-lineal, however the evidence for μυστήριον is early. How best do you weigh that? and on top of that the P46 is only what appears to be there . Anyway, the textual evidence (to me) seems slightly on the μαρτυριον side, but the principle of "the harder reading is more likely to be the correct reading" seems to favour μυστήριον.
it's a hard one, and I personally would probably weight it lower than a B, Metzger says:
From an exegetical point of view the reading μαρτύριον τοῦ θεοῦ, though well supported (ℵc B D G P Ψ 33 81 614 1739 Byz itd, g vg syrh copsa arm eth Origen al), is inferior to μυστήριον, which has more limited but early support in P46vid? ℵ* A C 88 436 itr, 61 syrp copbo Hippolytus Ambrosiaster Ephraem Ambrose Pelagius Augustine Antiochus. The reading μαρτύριον seems to be a recollection of 1.6, whereas μυστήριον here prepares for its usage in ver. 7
Comfort has an interesting entry
1 Corinthians 2:1
WH NU τὸ μυστήριον τοῦ θεοῦ
“the mystery of God”
P46vid ℵ✱ A C syrp copbo
NKJVmg RSVmg NRSV ESVmg NASBmg NIVmg TNIVmg NEBmg REBmg NJB NAB NLT HCSBmg NETmg
variant/TR το μαρτυριον του θεου
“the testimony of God”
ℵ2 B D F G Ψ 33 1739 Maj itb syrh copsa
KJV NKJV RSV NRSVmg ESV NASB NIV TNIV NEB REB NJBmg HCSB NET
UBS3 cites P46vid? in support of the NU text. The question mark follows “vid” because the editors were not sure that P46 contains the word μυστηριον (“mystery”). Having examined the actual papyrus, I can affirm that the reading is μυστηριον (“mystery”), not μαρτυριον (“testimony”), because the Greek letter eta, though partially broken, is visible before the final four letters—which are also visible (ριον). The one letter makes all the difference in determining the reading. UBS4 (as well as the Nestle text) now lists this papyrus as P46vid.
WH NU have uncontestable support from the earliest extant document, P46. Several other witnesses, both early and diverse, also support WH NU. But the same can be said for the variant reading. So how then do we solve the problem? Competent textual critics such as Zuntz (1953, [p. 486] 101) and Fee (1987, 88; 1992, 5-8) have argued that μυστηριον is a scribal emendation influenced by 2:7. Other scholars, such as Brown (1968, 48-49) and Metzger (TCGNT), have argued that μαρτυριον is a scribal emendation influenced by 1:6. Actually, one can draw upon the context of 1 Cor 1–2 to support either word, because Paul’s message in these chapters is that his mission was to testify only of Christ, who is the mystery of God. The immediate context seems to support “mystery,” because chapter 2 focuses on the need for believers to receive revelation from the Spirit of God to truly understand all the hidden, secret riches of God that are in Christ Jesus (see 2:7-16). In summary, the internal and external evidence for this reading is divided, so it is not easy to make a decision of which variant is original. This indecision is displayed in the array of modern English versions. Though most versions follow “testimony,” these same versions print “mystery” in the margin.
So he goes for μυστηριον - maybe I will look at the p46 images myself sometime!
Excellent point about the impact on doctrine Julie, Very important to remember and think about that