I've always found some issues with the main idea of the Baha'i faith, for a few different reasons.
The first is that they equate Krishna to many other prophets, as if somehow Krishna in either case is the main god or spokesperson or prophet of Hinduism as a monolithic religion. Hinduism has had many prophets throughout the ages, not a single one can be tied to it's founding, but many known and unknown throughout the ages.
He's not, he's the main figure for Viashnavas, which while the largest Hindu religion isn't the majority.
Here are some facts concerning the matter of Krishna in The Bahá’í Faith (and on the Souls we term ‘Manifestations of God’ in general), dear Kapalika:
#1, While Krishna is counted among the Divine Lights in our faith, nowhere in The Holy Writings of ANY of The Central Figures is He called ‘The Founder of Hinduism’, nor do they present Hinduism as this monolithic religion with one particular individual birthing it. As
@adrian009 noted, the opposite is what actually occurs. It is acknowledged that Hinduism is a collection of vastly different religious traditions with their own beliefs and practices, and multiple originators. Krishna is seen as one of them and a major one, no doubt.
#2, The whole ‘Founder of Hinduism’ criticism is sort of accurate, but not exact. It originates from a common perception of some Bahá’ís themselves concerning the Identities of some of the more commonly-listed Manifestations of God. Most of the Individuals listed commonly are, indeed, the originators of their respective traditions. It is correct to point out the mistake in identifying Krishna thusly, however.
This also brings up the second problem that I have with it, it seems to suggest that anyone who doesn't follow those religions somehow are the "main" or only "real" ones. Since they are not recognizing the legitimacy of other religions by naming their deities or prophets they are basically saying they are not "real" and don't have a connection to the same god. That, or it's like they don't acknowledge the other deities.
Again, I must agree with adrian: just because a specific religion or holy teacher is not explicitly mentioned does not constitute any denial of the Light from within it or them. In
Gleanings, Bahá’u’lláh had said,
“There can be no doubt whatever that the peoples of the world, of whatever race or religion, derive their inspiration from one heavenly Source, and are the subjects of one God. The difference between the ordinances under which they abide should be attributed to the varying requirements and exigencies of the age in which they were revealed. All of them, except a few which are the outcome of human perversity, were ordained of God, and are a reflection of His Will and Purpose.”
So, Bahá’u’lláh makes it unambiguously clear that (whether He knew of it or not) all religions flow from the same Divine Source for the same basic purpose: to educate humankind.
As for other deities, Bahá’ís conceive of them as that they are the products of the ways of peoples of various cultures to relate to the same Supreme Being.
Then my third issue is that it fails to recognize the inherent differences of the religions, their philosophies and theology. Some of the religions they say come from the same god are radically different enough to make such a claim highly suspect. Buddhism is very different from Islam, for example. Guatama Buddha is recorded as saying things that when compared to Islam clash very strongly. I am very sure many Muslim posters here can bring up texts contemporary to Muhammad that prove this, and Buddhists can bring up texts contemporary or close to Guatama as well. Guatama didn't even see god as relevant and was agnostic! So how could he be a prophet of a god?
I can only say I've talked to people of the Baha'i faith once in person, but many times online. I always get this sense that they want to fit round pegs into square holes, and sometimes don't seem to totally understand the various religions. I can understand that there are similarities, some might even be compatible or in some ways almost the same, depending on sect or otherwise... but that doesn't make them the same, and it's rather wrong to act like they became 'changed' over time but were all originally very alike or revised when we have contemporary records proving otherwise.
If God is so inept as to provide the truth the first time, why should we trust the newest Baha'i version either? On the surface it might seem like a nice way to "unite" all the different religions but it doesn't hold up after scrutiny. Even consider that religions like Hinduism and Buddhism are NOT monotheistic, why do people of the Baha'i faith try to act like they are?
See the quote from
Gleanings as well as my following comment thereon above concerning the differences between religions.
As for the second paragraph, my understanding is that it is not that the original Teachings become changed or revised, rather one of two things tends to happen:
#1, The Followers become so attracted to the “apparent reality” (i.e. what something says at face-value) of any given teaching that they become unwilling to accept any other way of understanding it.
OR
#2, The Followers of the Teachings build so much on top of the original Teachings that they become buried underneath, sometimes causing the Followers to lose sight of the Purpose of those Teachings (and even of the Teachings Themselves).
As for your inquiry concerning (again) differences between religions, I understand that those are the products of the different cultures and time periods in which they sprang forth into being.
As for your inquiry concerning God, there are two considerations worthy of note:
#1, The Bahá’í Faith itself had originated from within the milieu of religions such as Shi’a Islam, Christianity, Judaism, and Zoroastrianism, which are all monotheistic, so it, too, is a reflection of its own culture.
#2, One thing that Bahá’ís believe about God is that His Essence (who or what He is) can never be fully apprehended by human beings. All we'll ever have are our own conceptions built around Him which cannot hope to exhaust the Knowledge of Himself. So, what you're dealing with is, basically, varying conceptions of the same Ultimate Reality. As far as I'm concerned, I don't try to paint all religions as monotheistic. I am fully aware and very appreciative of the existence of different conceptions of the Ultimate Reality among the various religions of the world.