To those who have been once gathered to Christ he promises eternal life with him, by which statements the reveries of Origen and of the Chiliasts are abundantly refuted. For the life of believers, when they have once been gathered into one kingdom, will have no end any more than Christ’s. Now, to assign to Christ a thousand years, so that he would afterwards cease to reign, were too horrible to be made mention of. Those, however, fall into this absurdity who limit the life of believers to a thousand years, for they must live with Christ as long as Christ himself will exist. We must observe also what he says—we shall be, for he means that we profitably entertain a hope of eternal life, only when we hope that it has been expressly appointed for us. – John Calvin, Calvin’s Commentaries, 1 Th 4:15
Honestly, could you say it any better?