Messiah and Son of God

Jesus had two roles and claimed two titles in mortality - messiah / redeemer and Son of God

Why do we need a redeemer?

The purpose of God is to bring about the immortality and eternal life of man. Why? Man is that he may have joy. Spirits existed and God was the greatest, and He desired that we should be like Him.

How to do this? We needed corporeality (immortality), unity (Eternal life - God's life - my name is Eternal - be one as I and my Father are one - unity) and agency (to act and not be acted upon). Corporeality needs mortality. Thus, after the Creation, the Fall - a descent into mortality, by choice (2 Ne 2 concepts of agency are key - God being God could not force us to be like Him (D&C 121), He could only set up conditions where we could choose to be like Him, for He is an agent, and to be like Him, we must also be agents. 2 Ne 2:26.)

The Fall brings mortality, which is a state in which death occurs, which death is a separation from God and from each other. This is the opposite of God's desire, which is for no death, no separation. We are incapable of resurrecting, incapable of maintaining relationships forever, and are acted upon instead of being actors, and thus we need to be redeemed from the Fall. ("By proving contraries, truth is made manifest." History of the Church, 6:428 - God approached the work from a contrary position, in a way.)

That redemption must then reverse the Fall - avoiding the separation of death by reversing death, restoring to life permanently, and allowing relationships with others and God to be continued and perfected (Atonement), and giving us the chance to be agents, who act and who are not acted upon (immortality and eternal life).

That is what Jesus did for us as a Redeemer. His resurrection nullifies the Fall and makes it possible for the works of God to be accomplished.

How could He do this? As the Son of God (literal or adoptive? KF / SitG indicate that we are eternal, existing with God. Abraham seems to confirm this - spirits, not children. We become children of Israel, and of Christ by adoption, it seems reasonable that we might become children of God by adoption as well.). His Sonship must have been a prerequisite for achieving a redemption. He lived a sinless life (what does that mean exactly if sin = not-love?) and was a full agent, which we are not, capable of taking His life back, despite a descent into mortality, as a result of His pre-mortal divinity.

What a limited understanding I have - I haven't said it exactly right, but I have at least tried to approach it.