Can any of you please explain why God would choose to splinter Christendom into innumerable sects/denominations that have spent the last 500 years eroding the Christian unity that He promised to pray for?
John 17:20–21 (ASV): Neither for these only do I pray, but for them also that believe on me through their word; 21that they may all be one; even as thou, Father, art in me, and I in thee, that they also may be in us: that the world may believe that thou didst send me.
This is my opinionated answer and it is somewhat controversial/debatable:
This phenomena of the branching of
Christendom" is covered by Jesus in Matthew 13 - the mysteries of the kingdom
Matthew 13:11 He answered and said unto them, Because it is given unto you to know the mysteries of the kingdom of heaven, but to them it is not given.
Read the whole chapter
Specifically:
Matthew 13
31 Another parable put he forth unto them, saying, The kingdom of heaven is like to a grain of mustard seed, which a man took, and sowed in his field:
32 Which indeed is the least of all seeds: but when it is grown, it is the greatest among herbs, and becometh a tree, so that the birds of the air come and lodge in the branches thereof.
Jesus also said that the kingdom on earth would be a mixed multitude in the parable and explanation of the wheat and the tares.
Presumably the mustard seed tree parable is a focused historical summarized view of the mixed multitude of "Christendom" the various branches sprouting out of Christianity so-called with the birds being the forces of evil.
Of HIS Church Jesus said:
Matthew 16:18 And I say also unto thee, That thou art Peter, and upon this rock I will build my church;
and the gates of hell shall not prevail against it.
The last phrase showing that although the gates of hell shall not prevail against the kingdom, they will certainly try or else why did Jesus give the warning.
Why did He allow this?
For His glory of course.
Matthew 13
38 The field is the world; the good seed are the children of the kingdom; but the tares are the children of the wicked one;
39 The enemy that sowed them is the devil; the harvest is the end of the world; and the reapers are the angels.
40 As therefore the tares are gathered and burned in the fire; so shall it be in the end of this world.
41 The Son of man shall send forth his angels, and they shall gather out of his kingdom all things that offend, and them which do iniquity;
42 And shall cast them into a furnace of fire: there shall be wailing and gnashing of teeth.
43 Then shall the righteous shine forth as the sun in the kingdom of their Father. Who hath ears to hear, let him hear.