Our Lord Jesus explained this when He said,
Matt 20:16
16 So the last shall be first, and the first last: for many be called, but few chosen.
KJV
His Apostles were 'sent', which is what being a chosen apostle is about. Apostle Paul is one of the strongest examples, because Paul was persecuting Christ's Church when Jesus struck him down and chose him as His vessel to take The Gospel to the Gentiles (Acts 9).
To understand predestination per God's Word, one must first understand this difference between being called and being chosen. John 17 is an important Chapter in that study, as Jesus showed there His Apostles were already owned by The Father beforehand, and that they were 'sent' ones to preach The Gospel so that others called might believe and both would become One in The Father and The Son.
Matt.24:24 is also an important part of this, because it shows the deception the coming false messiah is to cause would, if it were possible, deceive even Christ's very elect. That phrase, "if it were possible" is a revealing that His chosen sent ones cannot... be deceived.
And then Christ's and His Apostle's preaching to the Church admonishing members to be careful to not be deceived or fall away is a revealing of how it is possible for the called to be deceived by Antichrist.
In essence, Christ's chosen are free from being tested by this world, for The Father already owned them before the foundation of this world. He directly influences their will and understanding. They will never be in danger of falling away, for they are the few that are chosen and sent.
But the rest of us who are called only, we must come to Christ of our own free will when called and choose to remain in Christ to the end, and thus we are tested.