The Mormon Church claims the Catholic Church changed the belief on the nature of God, which is just another sign that the Catholic Church lost apostolic authority.
The Mormon Church:
Scholars have long acknowledged that the view of God held by the earliest Christians changed dramatically over the course of centuries. Early Christian views of God were more personal, more anthropomorphic, and less abstract than those that emerged later during Christianity’s creedal stage. The key ideological shift that began in the second century, after the loss of apostolic authority, resulted from a conceptual merger of Christian doctrine with Greek philosophy.
Latter-day Saints believe the melding of early Christian theology with Greek philosophy was a grave error. Chief among the doctrines lost in this process was the nature of the Godhead. Latter-day Saints hold that God the Father is an embodied being with the attributes ascribed by the earliest Christians. That belief is consistent with the early Christian views of God, yet it differs from the later creeds.
Joseph Smith:
God himself. was Once as we are now, and is an exalted man, and sits enthroned in yonder heavens! That is the great secret. If the veil were rent today, and the great God who holds this world in its orbit, and who upholds all worlds and all things by His power, was to make himself visible,—I say, if you were to see him today, you would see him like a man in form like yourselves in all the person, image, and very form as a man;…
Lorenzo Snow:
As man now is, God once was; as God is now man may be.
When investigating the beliefs of the early church, Cardinal John Henry Newman points out in his book:
1 All the early church fathers (ECF) were not orthodox on every Catholic belief at every moment of Christian history. But looking at the group in total they were Catholic.
2 Anti-Catholics liked to quote the ECF to try and ‘prove’ the early church was not Catholic but that is not a good idea because while you could question Catholic teaching, you will never find unique non-Catholic teachings using the same standard.
3 Sometimes while quoting the ECF you are not quoting what they believe, but you are quoting them describing what the heretic, they are defending orthodoxy from, believes.
After his investigation, the Anglican priest, John Newman became Catholic, not Mormon.
When investigating apostolic authority Father Sullivan believes it was never lost. His book is a controversial walk through history to show where is was, but it was never lost.
TomNossor likes to invoke Cardinal Newman and Father Sullivan in the way Cardinal Newman described in point 2; as anti-Catholic poking the Catholic Church, but they would never support the Mormon claims.
God was never a human and the Mormon Church does not have apostolic authority. As the OP suggested, the Mormon Church can never provide any proof of their claims.
Richard Lyman Bushman, a Mormon historian, said, a Mormon’s “testimony” is their empirical evidence.
Basically Mormons just believe without proof.
I do not know what more I can say.
Prove the Mormon claims. Give some quotes from the ECF to show they believed God was once a man.