Why was Christ raised from among the dead?...

So we may be declared righteous in the sight of God.

The substitution of Christ’s life for our life was necessary in order to make an atonement for our sins. The innocent had to die for the guilty in order to balance the scales of Divine justice and righteousness.

The scales have been balanced (for had they not been balanced, righteousness could not have been ascribed to us under any circumstances). Since God the Father has raised Christ from the dead the way is now clear for ascribed righteousness to flow to us as individuals.

Upon what basis will righteousness be ascribed?

Righteousness will be ascribed on the basis of our faith in the promise of God, as was true of Abraham.

But on what promise will our faith rest? The promise that Christ died for our sins and that God raised Him again as an evidence that the sacrifice has been accepted. It remains to us to believe in God, and to demonstrate our belief by receiving the remission of sins that God has provided through the death of His Son.

If we will believe God has forgiven us through Christ, that Christ has been raised from the dead, and that we too will be raised from the dead, God will declare us righteous apart from the works of the Law just as He declared Abraham righteous apart from the works of the Law.