In 1 John 5:8, is the reference to spirit, water, and blood an allusion to Jesus' crucifixion?
In order to answer this question appropriately, we probably need to take from v 6 thru to and including v 12 (and then we include God's own witness bearing aspect regarding His Son), although some bibles, the KJV in particular, include an extra verse, with trinitarian implications, and identify it as the 7th verse, but this is not evident in the majority of original manuscripts (only in some, but these are primarily Latin manuscripts but excluding, at least the original text of, Jerome's "Latin Vulgate", see Wikipedia.) and is known as the "Johannine comma", although not really a comma as such. So in the interests of completeness, I include narrative on this controversial verse, particularly here as there is a "bounty" involved.
1 John 5:7 in the KJV reads as follows:-
"For there are three that beare record [ in heaven, the Father, the Word, and the Holy Ghost: and these three are one.]"
The "Johannine comma" is a textual variant in regards to 1 John 5:7-8. The word comma simply means "short clause," and Johannine means "pertaining to John." Without the "comma," 1 John 5:7-8 reads,
"For there are three witness bearers, the spirit, and the water and the blood, and the three are in agreement." NWT
Faith in Jesus Christ as the savior of mankind , through his "Ransom Sacrifice", is what's at stake for earthly man here. What is not at stake is mankind's belief in a triune God. Consequently, I am of the opinion that the KJV's verse's authenticity, to remain, is questionable. What we would seem to have here is a possible case of -trinitarian leaning - translators bias, taking advantage of the 3 in 1 (in agreement) concept, in order to convey their trinitarian dogma.
If we take verses 5 through 8, apart from the "extended version" of verse 7, who and what are we talking about after all. We are talking about Jesus Christ, with regard to the "who". With regard to the "what", we are talking about Jesus' (in agreement) witness bearers, the three:- noun-nominative neuter singulars, i.e. the spirit and the water and the blood. Consequently, we have the truth (spirit); the baptism (water); and the blood (ransom sacrifice), period. All the earthly attributes, with regard to the Son of Man. Attributes, for earthly man to consider and understand, as having been manifested out/by way of, the Son of God, through faith in - all that is encompassed by - the "Ransom Sacrifice".
Having said this, although the "blood" is a definitive reference to the "Ransom Sacrifice", I don't necessarily see the "spirit" and the "water", as portrayed in 1 John 5:8, as being also an allusion to Jesus' crucifixion, as per John 19:34-35.