Many exegetes claim, without proof, that Jesus resurrected before the angel rolled back the stone, in order to show that he passed through it as he could walk through doors or as he was born out of the Blessed Virgin Mother's closed, virginal womb.
Mt. 28:2 And behold there was a great earthquake. For an angel of the Lord descended from heaven, and coming, rolled back the stone, and sat upon it.
St. Thomas Aquinas, Commentary on St. Matthew, ch. 28:
Christ had already risen: for as He went out from a closed womb, so He went out from the sealed tomb.
Ven. Bede, as quoted by St. Thomas in Catena in Mt. cap. 28 l. 1:
"And came and rolled back the stone;" not to open the door for the Lord to come forth, but to give evidence to men that He was already come forth. For He who as mortal had power to enter the world through the closed womb of a Virgin, He when become immortal, was able to depart out of the world by rising from a sealed sepulchre.
Cornelius à Lapide, S.J., Commentary on Matthew 22-28, ch. 28:
And rolled away the stone, of the sepulchre of Christ. Not that He might rise out of it, for He had already risen while the sepulchre was closed, but that he might show to the women that Christ their God and Lord had already risen. Opening to them a way to the sepulchre, he showed it to them empty of the body of Christ. For as Christ was born from the closed womb of the Virgin, so also He rose from the closed sepulchre.
But what evidence is there that the Resurrection occurred before the angel rolled back the stone?