Because the Bible tells us so:
Romans 8:16-17 (NAB)
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The Spirit itself bears witness with our spirit that we are children of God,
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and if children, then heirs, heirs of God and joint heirs with Christ, if only we suffer with him so that we may also be glorified with him.
You do realize that the Catechism also says that one can become a joint/co heir with Christ?
2009 Filial adoption, in making us partakers by grace in the divine nature, can bestow true merit on us as a result of God’s gratuitous justice. This is our right by grace, the full right of love, making us “co-heirs” with Christ and worthy of obtaining “the promised inheritance of eternal life.” The merits of our good works are gifts of the divine goodness. “Grace has gone before us; now we are given what is due. . . . Our merits are God’s gifts.”
1 God, infinitely perfect and blessed in himself, in a plan of sheer goodness freely created man to make him share in his own blessed life. For this reason, at every time and in every place, God draws close to man. He calls man to seek him, to know him, to love him with all his strength. He calls together all men, scattered and divided by sin, into the unity of his family, the Church. To accomplish this, when the fullness of time had come, God sent his Son as Redeemer and Savior. In his Son and through him, he invites men to become, in the Holy Spirit, his adopted children and thus heirs of his blessed life.
Among many others.
We do not know the specifics of this, as it has not been revealed.
I do not think that this devalues the sacrifice of Jesus Christ at all. Jesus Christ is the Savior for all of us. Whether or not the Father was a Savior in some other existence has no relevance or bearing on Christ being our Savior, and that it is only through Him that we are saved.