The resurrection was necessary for our justification because His resurrection was the basis of us being born again (1Pet. 1:3). Because His resurrection declared Him to be the victorious Risen Lord of all. Wayne Grudem explains this verse this way:
If God “raised us up with him” (Eph. 2:6), then, by virtue of our Union With Christ, God’s declaration of approval of Christ is also his declaration of approval of us. When the Father in essence said to Christ, “All the penalty for sins has been paid and I find you not guilty but righteous in my sight,” he was thereby making the declaration that would also apply to us once we trusted in Christ for salvation. In this way Christ’s resurrection also gave final proof that he had earned our justification.
John Gill writes:
was raised again for our justification; he was raised again from the dead by his Father, to whom this is often ascribed; and by himself, by his own power, which proves him to be the mighty God; and this was done not only that he might live an immortal and glorious life in our nature, having finished the work he undertook and came about, but for "our justification". He died in the room and stead of his people, and by dying made satisfaction for their sins; he rose again as their head and representative, and was legally discharged, acquitted, and justified, and they in him. Christ's resurrection did not procure the justification of his people, that was done by his obedience and death; but was for the testification of it, that it might fully appear that sin was atoned for, and an everlasting righteousness was brought in; and for the application of it, or that Christ might live and see his righteousness imputed, and applied to all those for whom he had wrought it out.
Ellicott observes the connection between His death and resurrection for our justification:
The death of Christ is the proper cause of justification, or means of atonement, according to St. Paul; the resurrection of Christ is only the mediate or secondary cause of it. The atoning efficacy lay in His death, but the proof of that efficacy—the proof that it was really the Messiah who died—was to be seen in the Resurrection. The Resurrection, therefore, gave the greatest impulse to faith in the atoning efficacy of the death upon the cross, and in this way helped to bring about justification. Comp. especially 1Co. 15:17, “If Christ be not raised, your faith is vain; ye are yet in your sins”—i.e., you have no guarantee that your sins have really been remitted; if the death of Christ had not been followed by His resurrection, the inference would have followed that it was merely the death of an ordinary man, and without any special saving efficacy.
Here I end our brief study of Christ's resurrection.
The Ascension of Christ
Luke tells us that the Lord Jesus taught His disciples about the Kingdom of God for forty days between His resurrection and ascension. With the same body, He blessed His disciples and was taken up from them:
Acts 1:8-11 But you will receive power when the Holy Spirit has come upon you, and you will be my witnesses in Jerusalem and in all Judea and Samaria, and to the end of the earth.” 9 And when he had said these things, as they were looking on, he was lifted up, and a cloud took him out of ...