"Firstly, if what you say was actually true, it would mean that God was not exactly being fair or just to everyone, if He did not actually send a Divine Spiritual Teacher / Christ to each and every generation of mankind. "
Piffle.
The teachings of Christ were good for a thousand years-Muhammad for another millenia. Just because the Manifestation cannot be seen by everyone does not mean His message can't be heard by everyone.
And as to what God does and does not do: "O Shaykh! Every time God the True One -- exalted be His glory -- revealed Himself in the person of His Manifestation, He came unto men with the standard of "He doeth what He willeth, and ordaineth what He pleaseth." None hath the right to ask why or wherefore, and he that doth so, hath indeed turned aside from God, the Lord of Lords.
(Baha'u'llah, Epistle to the Son of the Wolf, p. 67)
"The spirit that animateth the human heart is the knowledge of God, and its truest adorning is the recognition of the truth that "He doeth whatsoever He willeth, and ordaineth that which He pleaseth." Its raiment is the fear of God, and its perfection steadfastness in His Faith. Thus God instructeth whosoever seeketh Him. He, verily, loveth the one that turneth towards Him. There is none other God but Him, the Forgiving, the Most Bountiful. All praise be to God, the Lord of all worlds."
(Baha'u'llah, Gleanings from the Writings of Baha'u'llah, p. 290)
"He it is Who is the manifestation of "God
doeth whatsoever He pleaseth", and abideth upon the
throne of "He ordaineth whatsoever He chooseth".
(Baha'u'llah, The Kitab-i-Aqdas, p. 76)
"For these hidden Gems, these concealed and invisible Treasures, in themselves manifest and vindicate the reality of these holy words: "Verily God doeth whatsoever He willeth, and ordaineth whatsoever He pleaseth." 98
To every discerning and illumined heart it is evident that God, the unknowable Essence, the divine Being, is immensely exalted beyond every human attribute, such as corporeal existence, ascent and descent, egress and regress. Far be it from His glory that human tongue should adequately recount His praise, or that human heart comprehend His fathomless mystery. He is and hath ever been veiled in the ancient eternity of His Essence, and will remain in His Reality everlastingly hidden from the sight of men. "No vision taketh in Him, but He taketh in all vision; He is the Subtile, the All-Perceiving."[1] No tie of direct intercourse can possibly bind Him to His creatures. He standeth exalted beyond and above all separation and union, all proximity and remoteness. No sign can indicate His presence or His absence; inasmuch as by a word of His command all that are in heaven and on earth have come to exist, and by His wish, which is the Primal Will itself, all have stepped out of utter nothingness into the realm of being, the world of the visible."
[1 Qur'án 6:103.]
(Baha'u'llah, The Kitab-i-Iqan, p. 97)
"Gracious God! So great is the folly and perversity of the people, that they have turned their face toward their own thoughts and desires, and have turned their back upon the knowledge and will of God -- hallowed and glorified be His name!
Be fair: Were these people to acknowledge the truth of these luminous words and holy allusions, and recognize God as "Him that doeth whatsoever He pleaseth," how could they continue to cleave 172 unto these glaring absurdities? Nay, with all their soul, they would accept and submit to whatsoever He saith. I swear by God! But for the divine Decree, and the inscrutable dispensations of Providence, the earth itself would have utterly destroyed all this people! "He will, however, respite them until the appointed time of a known day.""
(Baha'u'llah, The Kitab-i-Iqan, p. 171)
"Whenever Zachariah entered the chamber to her he found beside her a provision, and said, 'O Mary, how hast thou this?' She said, 'It is from God, for God provides for whom He pleases without count.' Therefore prayed Zachariah to his Lord, and said, 'Lord, grant me from Thee a good seed. Verily, Thou hearest prayer.' And an angel cried out to him as he was standing praying in the chamber (and said) that 'God gives thee the glad tidings of John, to confirm the Word from God,- of a chief and a chaste one, and a prophet from amongst the righteous.'
He said, 'My Lord, how can there be to me a boy when old age has reached me, and my wife is barren?' Said he, 'Thus God does what He pleaseth.' He said, 'My Lord, make for me a sign.' He said, 'Thy sign is that thou shalt not speak to men for three days, save by gesture; but remember thy Lord much, and celebrate His praises in the evening and the morning.'"
(The Qur'an (E.H. Palmer tr), Sura 3 - Imran's Family)
"There is not a beast upon the earth nor a bird that flies with both its wings, but is a nation like to you; we have omitted nothing from the Book; then to their Lord shall they be gathered. Those who say our signs are lies- deafness, dumbness, in the dark! whom He pleases does God lead astray, and whom He pleases He places on the right way."
(The Qur'an (E.H. Palmer tr), Sura 6 - Cattle)
"Do they not see that God, whosoever is in the heavens adores Him, and whosoever is in the earth, and the sun, and the moon, and the stars, and the mountains, and the beasts, and many among men, though many a one deserves the torments?
Whomsoever God abases there is none to honour him; verily, God does what He pleases."
(The Qur'an (E.H. Palmer tr), Sura 22 - The Pilgrimage)
Finally there is no other greater justice than God's
"Were the mysteries, that are known to none except God, to be unraveled, the whole of mankind would witness the evidences of perfect and consummate justice. With a certitude that none can question, all men would cleave to His commandments, and would scrupulously observe them. We, verily, have decreed in Our Book a goodly and bountiful reward to whosoever will turn away from wickedness and lead a chaste and godly life. He, in truth, is the Great Giver, the All-Bountiful."
(Baha'u'llah, Gleanings from the Writings of Baha'u'llah, p. 117)
God does as He pleases and whatever He does is Just because God is never unjust.
Regards,
Scott