Mosiah 15

In much of Isaiah's writings, I find much of his words can be rightfully interpreted many ways

As Abinadi explains the words of Isaiah, he uses the word "Father" which could easily be interpreted as God the Father of our spirits, and the Savior, being the Father of heaven and earth and of salvation.  As Abinadi speaks of the Father and the Son before the wicked priests of Noah, it becomes very clear (a least to me)of whom he is speaking as he declares -

"...thus becoming the Father and Son—
4 And they are one God, yea, the very Eternal Father of heaven and of earth."  (Book of Mormon | Mosiah 15:3 - 4)

Even though we have a Father in Heaven who is God and was previously before our spirit existence, Abinadi declares of the Savior that He would "become" the Father of salvation.