In Catholicism, the belief is that God has eternally existed as three distinct Persons: Father, Son, and Holy Ghost. There never was a time when one Person did not exist, nor a time when they have not existed in the eternal relationship of the Trinity, nor has there been a time when God (whether collectively or an individual Person of the Trinity) did not exist as fully Divine, fully God. The Incarnation of God the Son did not result in a change in the nature of the Divine. Indeed, traditional Christians believe that man and God are of two different natures. Therefore, the Incarnation of the Son resulted in an addition of human nature to the Person of the Son. Further, the sanctification of the saints that results in deification or theosis is accomplished through the grace of God. As St. Athanasius taught, we become by grace what God is by nature. St. Thomas also taught, in his Summa, that “the full participation of the Divinity, [is] the true bliss of man and end of human life; and this is bestowed upon us by Christ’s humanity”.
Mormon theology allows for the belief that the Father was not always God. Indeed, Joseph Smith, in the King Follett Discourse, said that he would “refute that idea” that God was always God from all eternity (which is the Catholic/Orthodox view). Various other leaders, as well as LDS Church manuals and magazines, have discussed the matter of the Father progressing to Godhood, that God became God, and that we follow His example:
In an older edition of the Gospel Principles manual, we read:
***The Prophet Joseph Smith taught: “ When you climb up a ladder, you must begin at the bottom, and ascend step by step, until you arrive at the top; and so it is with the principles of the Gospel — you must begin with the first, and go on until you learn all the principles of exaltation. But it will be a great while after you have passed through the veil [died] before you will have learned them. It is not all to be comprehended in this world; it will be a great work to learn our salvation and exaltation even beyond the grave ” ( Teach- ings of the Prophet Joseph Smith, p. 348).
This is the way our Heavenly Father became God. Joseph Smith taught: “ It is the first principle of the Gospel to know for a certainty the character of God … He was once a man like us ;…God himself, the Father of us all, dwelt on an earth, the same as Jesus Christ himself did ” ( Teachings of the Prophet Joseph Smith, pp. 345 – 46). Our Heavenly Father knows our trials, our weaknesses, and our sins. He has compassion and mercy on us. He wants us to succeed even as he did.***
lds.org/gospellibrary/materials/gospel/Start%20Here_01.pdf (“Exaltation” chapter)
In the Gospel Fundamentals manual, we read:
"Becoming like our Father in Heaven is like climbing a ladder. We must start at the bottom and climb each step until we reach the top. The Prophet Joseph Smith said that if we want to become like our Father in Heaven we must learn how He feels, thinks, and acts. When we understand these things about Him, we can then learn all other things about Him, until we know how to become as He is.
It will help us to remember that our Father in Heaven was once a man who lived on an earth, the same as we do. He became our Father in Heaven by overcoming problems, just as we have to do on this earth. However, the Prophet Joseph Smith said we will not learn everything we need to learn while in this world. It will take us a long time after we complete this life to know all the things we need to know in order to become like our Father in Heaven."***
In the Teachings of Presidents of the Church: Brigham Young manual, we read:
"God the Father, His Son Jesus Christ, and the Holy Ghost constitute the Godhead. President Brigham Young taught the Latter-day Saints to worship God the Father and address prayers to Him in the name of Jesus Christ. He taught further that God the Father was once a man on another planet who “passed the ordeals we are now passing through; he has received an experience, has suffered and enjoyed, and knows all that we know regarding the toils, sufferings, life and death of this mortality”"
"The great architect, manager and superintendent, controller and dictator [absolute ruler] who guides this work is out of sight to our natural eyes. He lives on another world; he is in another state of existence; he has passed the ordeals we are now passing through; he has received an experience, has suffered and enjoyed, and knows all that we know regarding the toils, sufferings, life and death of this mortality, for he has passed through the whole of it, and has received his crown and exaltation and holds the keys and the power of this Kingdom; he sways his scepter, and does his will among the children of men, among Saints and among sinners, and brings forth results to suit his purpose among kingdoms and nations and empires, that all may redound to his glory and to the perfection of his work (DBY, 22)."
"The doctrine that God was once a man and has progressed to become a God is unique to this Church. How do you feel, knowing that God, through His own experience, “knows all that we know regarding the toils [and] sufferings” of mortality?"
There are of course many other examples of the inclusion of the belief of the Father progressing to Godhood, and not always having been God, within LDS teaching. This is somewhat different from the LDS belief that Jesus was a God prior to Him coming to the earth, and that He was the Jehovah of the Old Testament.