Read 1 John 5:7-8 in the oldest Greek manuscripts in context as if never read before and it explains itself.
John is not making modern theological claims that modern theology seems to require.
John is clearly talking about the evidence of Who JESUS is as the Holy Son of God.
He did not come only by water.
Clearly this would be the normal natural human birth by water.
He came by water AND blood.
The blood is spoken of in chapter 1 by which we are purified, thus this is clearly referring to the extraordinary facet of His coming as the Holy Lamb of God.
And finally, the Spirit clearly testifies that JESUS is the One.
He was born from above (Spirit)
He was born in humanity (water)
He was born as the pure Lamb (blood)
Wisdom in the WORD 