They never left Earth but have been in fact been preserved by virtue of living in the presence of the Tree of Life - which exists to this day although is off limits to us.
Now they have returned to do spiritual battle against the armies of the Antichrist. They also have the power to call down fire and chastisements upon the wicked. But their primary mission is to prepare the way for the Second Coming, just as John the Baptist prepared the way for the First Coming.
They make their way to Jerusalem where their mission will be to compete with the Antichrist for the souls of men:
"The conversion of the Jews is to be the special work of Elijah, and that of the Gentiles of Enoch; for which purpose these two prophets will be peculiarly adapted, Enoch, the seventh from Adam, representing the uncircumcision, and Elijah the circumcision. The period during which Enoch and Elijah are to exercise their office is one thousand two hundred and sixty days; a space of time corresponding with the duration of Christ's public ministry.
During this time they will be in direct competition with the Antichrist for the souls of men. The purpose of God in sending them is to strengthen the faith of those who already believe, and to give those who do not one last chance to accept Jesus as the Messiah before the Final Judgment. The Fathers and Doctors of the Church concur:
St. Ephraem (306 - 373 AD) "And when the Son of Perdition has drawn to his purpose the whole world, Enoch and Elijah shall be sent that they might confute the Evil One."
St. Augustine (354 - 430 AD) "It is a familiar theme in the conversation and heart of the faithful, that in the last days before the Judgment the Jews shall believe in the true Christ, that is, our Christ, by means of this great and admirable prophet Elijah, who shall expound the Law to them. For not without reason do we hope that before the coming of our Judge and Saviour, Elijah shall come, because we have good reason to believe that he is now alive. For, as Scripture most distinctly informs us, he was taken up from this life in a chariot of fire. When, therefore, he is come, he will give a spiritual explanation of the Law which the Jews at present understand carnally, and shall thus, "turn the heart of the father to the son", that is, the heart of the fathers to the children. And the meaning is that the sons, that is, the Jews, shall understand the Law as the fathers, that is, the prophets, and among them Moses himself, understood it...that the Jews also, who had previously hated, should then love the Son who is our Christ."
St. John Damascene (676 - 749 AD) "And Enoch and Elijah the Thesbite will be sent and they shall 'turn the heart of the fathers to the children', that is to say, turn the Synagogue to our Lord Jesus Christ and the preaching of the Apostles. And they will be destroyed by the Antichrist."
St. Robert Bellarmine (1542 - 1621 AD) "The third demonstration arises from the coming of Enoch and Elijah, who live even now and shall live until they come to oppose Antichrist himself, and to preserve the elect in the faith of Christ, and in the end shall convert the Jews, and it is certain that this has not yet been fulfilled."
Using the writings of St. Thomas Aquinas, Suarez, and Tertullian, Fr. Charles Garside gives us a fascinating analysis of the vision spoken of by St. John in the Book of the Apocalypse. From this we can form a clearer picture of what things will be like when the Two Witnesses take to the field of battle against the Man of Lawlessness: