In a moment - (ἐν ἀτόμῳ en atomō). In an “atom,” scil. of time; a point of time which cannot be cut or divided (α a, the alpha privative (“not”) and τομη tomē, from τέμνω temnō, “to cut”). A single instant; immediately. It will be done instantaneously.
In the twinkling of an eye - This is an expression also denoting the least conceivable duration of time. The suddenness of the coming of the Lord Jesus is elsewhere compared to the coming of a thief in the night; 2 Peter 3:10. The word rendered “twinkling” (ῥιπῆ ripē, from ῥίπτω rhiptō, “to throw, cast”) means “a throw, cast, jerk,” as of a stone; and then “a jerk of the eye,” that is, “a wink” - Robinson.
At the last trump - When the trumpet shall sound to raise the dead. The word “last” here does not imply that any trumpet shall have been before sounded at the resurrection, but is a word denoting that this is the consummation or close of things; it will end the economy of this world; it will be connected with the last state of things.
For the trumpet shall sound - See the note at Matthew 24:31.
And the dead shall be raised - See the note at John 5:25.