John 16:17-18 Then some of His disciples said among themselves, “What is this that He says to us, ‘A little while, and you will not see Me; and again a little while, and you will see Me’; and, ‘because I go to the Father’?” They said therefore, “What is this that He says, ‘A little while’? We do not know what He is saying.”
Why couldn't the disciples of Jesus accept that Jesus must be killed, and then be resurrected from the dead (see them again) as prophesied?
Isa 53:5-7 But He was wounded for our transgressions, He was bruised for our iniquities; the chastisement for our peace was upon Him, and by His stripes we are healed. All we like sheep have gone astray; we have turned, every one, to his own way; and the Lord has laid on Him the iniquity of us all. He was oppressed and He was afflicted, yet He opened not His mouth; He was led as a lamb to the slaughter, and as a sheep before its shearers is silent, so He opened not His mouth.
The Jews only wanted their Messiah to be the King of Israel and refused to accept that He was also the sacrificial Lamb of God. Though our Lord reminded His disciples again and again that He would come as the sacrificial Lamb of God (testified by their prophet John the Baptist), they still refused to accept it.
They confessed Him as Christ'
John 6:68-69 But Simon Peter answered Him, “Lord, to whom shall we go? You have the words of eternal life. Also we have come to believe and know that You are the Christ, the Son of the living God.”
Though they confessed Jesus as the Christ, the Son of the living God earlier they were not saved for they did not receive the Holy Spirit as the seal of salvation. No wonder they were not able to understand that the Messiah must be killed and then be resurrected from the dead.
John 20:8-9 Then the other disciple, who came to the tomb first, went in also; and he saw and believed. For as yet they did not know the Scripture, that He must rise again from the dead.
Luke 24:6-8 "He is not here, but is risen! Remember how He spoke to you when He was still in Galilee, saying, ‘The Son of Man must be delivered into the hands of sinful men, and be crucified, and the third day rise again.’” And they remembered His words.
The disciples were always told and reminded that Jesus must be crucified and then be resurrected so that they would believe Him as the sacrificial Lamb of God.
Luke 24:45-47 And He opened their understanding, that they might comprehend the Scriptures. Then He said to them, “Thus it is written, and thus it was necessary for the Christ to suffer and to rise from the dead the third day, and that repentance and remission of sins should be preached in His name to all nations, beginning at Jerusalem.
After they had understood the gospel and believed that Jesus was their sacrificial Lamb who bore their repented sins on the cross, died and then be resurrected from the dead for the remission of their sins then they were saved. Immediately our Lord Jesus gave them the Holy Spirit as the seal of salvation.
John 20:21-21 So Jesus said to them again, “Peace to you! As the Father has sent Me, I also send you.” And when He had said this, He breathed on them, and said to them, “Receive the Holy Spirit."
Brethren, we must preach the gospel as taught and commissioned by our Lord Jesus Christ (in Luke 24:45-47) in its wholeness, otherwise there will be no remission of sins without the sacrificial Lamb, nor salvation with diluted gospel by televangelists, prosperity gospel, etc.