John Calvin, in his commentary on the synoptic gospels (Matthew, Mark, Luke), writes this with respect to Mark 13:22/Matthew 24:24 ("False christs and false prophets will arise and perform signs and wonders, to lead astray, if possible, even the elect."):
"This was added for the purpose of exciting alarm, that believers may be more careful to be on their guard; for when such unbounded freedom of action is allowed to false prophets, and when they are permitted to exert such powers of deceiving, those who are careless and inattentive would easily be entangled by their snares. Christ therefore exhorts and arouses his disciples to keep watch, and at the same time reminds them that there is no reason for being troubled at the strangeness of the sight, if they see many persons on every hand led away into error. While he excites them to solicitude, that Satan may not overtake them in a state of sloth, he gives them abundant ground of confidence on which they may calmly rely, when he promises that they will be safe under the defense and protection of God against all the snares of Satan. And thus, however frail and slippery the condition of the godly may be, yet here is a firm footing on which they may stand; for it is not possible for them to fall away from salvation, to whom the Son of God is a faithful guardian. For they have not sufficient energy to resist the attacks of Satan, unless in consequence of their being the sheep of Christ, which none can pluck out of his hand (John 10:28). It must therefore be observed, that the permanency of our salvation does not depend on us, but on the secret election of God; for though our salvation is kept through faith, as Peter tells us (1 Peter 1:5), yet we ought to ascend higher, and assure ourselves that we are in safety, because the Father hath given us to the Son, and the Son himself declares, that none who have been given to him shall perish (John 17:12)." Harmony of the Evangelists, Vol. 3, p. 141
P.S. I notice that in Mark 14:35 and Matthew 26:39 this same phrase in Greek for "if possible" (ei dunaton) appears in Jesus' prayer in Gethsemane: "My Father, IF POSSIBLE, let this cup pass from me; nevertheless, not as I will, but as you will." Here is another example of something that is impossible and will never come about from the perspective of God's sovereign will, yet that exerts real influence as warning or prayer.