It was necessary for Christ to rise again, for five reasons.
First of all, for the commendation of divine justice, to which belongs exaltation of those who humble themselves for God's sake, according to Luke 1: "He deposes potentates from their seats and exalts the humble." Therefore because Christ, according to charity and obedience to God, humbled himself even to death on the cross, it was needful that he be exalted by God even to glorious resurrection, as it is said in His Person in the Psalm (138:2) as the Gloss expounds it, "You have known," that is, approved, "my sitting down," that is, humility and passion, "and my rising up," that is, glorification in resurrection.
Second, for our instruction in faith. Because through his resurrection our faith about Christ's divinity is confirmed, because, as is said at the end of II Corinthians, "Although he was crucified from our infirmity, he lives from God's power." And likewise, it is said in I Corinthians 15, "If Christ did not rise, our preaching is empty, and our faith is empty." And in the Psalm (29:10), "What profit is in my blood," that is, in the shedding of my blood, "while I descend," as it were through various grades of evil, "into corruption?" As though He were to answer, "None, If therefore I do not rise again at once, an my body be decayed, I shall bring news to no one, I shall profit no one," as the Gloss expounds.
Third, for the uplifting of our hope. Because, while we see Christ, who is our Head, rise again, we also hope in our own resurrection. Wherefore it is said in I Corinthians 15, "If Christ is preached that He rises from the dead, how is it said among you that there is no resurrection of the dead?" And in Job 19 it is said, "I know," that is, through the certainty of faith, "that my redeemer," that is, Christ, "lives," having risen from the dead, and therefore "in the last day, I shall rise out of the earth; this my hope is stored in my bosom."
Fourth, for the structuring (informationem) of the lives of the faithful, according to which [it is said in] Romans 6, "As Christ is risen from the dead through the glory of the Father, even so may we walk in newness of life." And further down, "Christ, rising from the dead, therefore no longer dies; so also recognize that you are dead to sin, but living to God.
Fifth, for the completion of our salvation. Because just as for this reason he endured evil things in dying so that he might liberate us from evil, so he is glorified in rising again so that he might promote us to good, according to Romans 4, "He was delivered for our sins, and he rose for our justification."
Thomas Aquinas, ST 3.53.1, my translation.