I think the key phrase is "if it were possible." Here is more context for those without a Bible handy:

15 “Therefore when you see the ‘abomination of desolation,’ spoken of by Daniel the prophet, standing in the holy place” (whoever reads, let him understand), 16 “then let those who are in Judea flee to the mountains. 17 Let him who is on the housetop not go down to take anything out of his house. 18 And let him who is in the field not go back to get his clothes. 19 But woe to those who are pregnant and to those who are nursing babies in those days! 20 And pray that your flight may not be in winter or on the Sabbath. 21 For then there will be great tribulation, such as has not been since the beginning of the world until this time, no, nor ever shall be. 22 And unless those days were shortened, no flesh would be saved; but for the [c]elect’s sake those days will be shortened. 23 “Then if anyone says to you, ‘Look, here is the Christ!’ or ‘There!’ do not believe it. 24 For false christs and false prophets will rise and show great signs and wonders to deceive, if possible, even the elect. 25 See, I have told you beforehand. 26 “Therefore if they say to you, ‘Look, He is in the desert!’ do not go out; or ‘Look, He is in the inner rooms!’ do not believe it. 27 For as the lightning comes from the east and flashes to the west, so also will the coming of the Son of Man be. 28 For wherever the carcass is, there the eagles will be gathered together.

The MacArthur study bible says this about verse 24:

to deceive, if possible, even the elect. This clearly implies that such deception is not possible (John 10:4, 5).

The ESV Reformation Study Bible says:

if possible. Although the false prophets try to deceive the elect, there is no real possibility that they will succeed. God will keep the elect secure in His love (Rom. 8:31–39; cf. John 10:28, 29). See “Perseverance of the Saints”at Rom. 8:30.

The King James Study Bible says:

The phrase if it were possible . . . shall deceive the very elect clearly indicates that those who have been truly saved cannot be deceived and fall away. For even if it were humanly possible, the Lord will stop it by hastening His coming.

John 10:1-6:

Most assuredly, I say to you, he who does not enter the sheepfold by the door, but climbs up some other way, the same is a thief and a robber. 2 But he who enters by the door is the shepherd of the sheep. 3 To him the doorkeeper opens, and the sheep hear his voice; and he calls his own sheep by name and leads them out. 4 And when he brings out his own sheep, he goes before them; and the sheep follow him, for they know his voice. 5 Yet they will by no means follow a stranger, but will flee from him, for they do not know the voice of strangers.” 6 Jesus used this illustration, but they did not understand the things which He spoke to them.

That, if it were possible, but of course it isn’t.

So the elect won’t be deceived. Not all who call themselves Christians are part of the elect (actual Christians).