The word “still” which is translated as “As yet” in (AV) indicates that eventually they would come to know. It is uncertain, however, exactly which scripture to which John is referring, much like Paul in (1 Cor 15:4). They could have in mind any of these: (Psa 16:10, Isa 53:10-12, Hos 6:2, Jonah 1:17). The important thing here, however, is that the “belief in the resurrection came first. The believers did not manufacture a resurrection to agree with their interpretation of prophecy.” (Morris)

1 Corinthians 15:4  that he was buried, that he was raised on the third day according to the Scriptures,

Psalms 16:10  because you will not abandon me to the grave, nor will you let your Holy One see decay.

Isaiah 53:10-12  Yet it was the LORD’s will to crush him and cause him to suffer, and though the LORD makes his life a guilt offering, he will see his offspring and prolong his days, and the will of the LORD will prosper in his hand.  After the suffering of his soul, he will see the light of life and be satisfied; by his knowledge my righteous servant will justify many, and he will bear their iniquities.  Therefore I will give him a portion among the great, and he will divide the spoils with the strong, because he poured out his life unto death, and was numbered with the transgressors. For he bore the sin of many, and made intercession for the transgressors.

Hosea 6:2  After two days he will revive us; on the third day he will restore us, that we may live in his presence.

Jonah 1:17  But the LORD provided a great fish to swallow Jonah, and Jonah was inside the fish three days and three nights.

John states that Christ “had to” rise from the dead. The necessity of this happening is well recorded (Mark 8:31; Matt 26:54; Luke 9:22; 17:25; 22:37; 24:7, 26, 44, John 3:14; 12:34; Acts 1:16). Interestingly and ironically, it is only the enemies of Jesus who recalled His words that He must rise again (Matt 27:62-64).

Mark 8:31  He then began to teach them that the Son of Man must suffer many things and be rejected by the elders, chief priests and teachers of the law, and that he must be killed and after three days rise again.

Matthew 26:54  But how then would the Scriptures be fulfilled that say it must happen in this way?”

Luke 9:22  And he said, “The Son of Man must suffer many things and be rejected by the elders, chief priests and teachers of the law, and he must be killed and on the third day be raised to life.”

Luke 17:25  But first he must suffer many things and be rejected by this generation.

Luke 22:37  It is written: ‘And he was numbered with the transgressors’; and I tell you that this must be fulfilled in me. Yes, what is written about me is reaching its fulfilment.”

Luke 24:7  ‘The Son of Man must be delivered into the hands of sinful men, be crucified and on the third day be raised again.’”

Luke 24:26  Did not the Christ have to suffer these things and then enter his glory?”

Luke 24:44  He said to them, “This is what I told you while I was still with you: Everything must be fulfilled that is written about me in the Law of Moses, the Prophets and the Psalms.”

John 3:14  Just as Moses lifted up the snake in the desert, so the Son of Man must be lifted up,

John 12:34  The crowd spoke up, “We have heard from the Law that the Christ will remain for ever, so how can you say, ‘The Son of Man must be lifted up’? Who is this ‘Son of Man’?”

Acts 1:16  and said, “Brothers, the Scripture had to be fulfilled which the Holy Spirit spoke long ago through the mouth of David concerning Judas, who served as guide for those who arrested Jesus—

Matthew 27:62-64  The next day, the one after Preparation Day, the chief priests and the Pharisees went to Pilate.  “Sir,” they said, “we remember that while he was still alive that deceiver said, ‘After three days I will rise again.’  So give the order for the tomb to be made secure until the third day. Otherwise, his disciples may come and steal the body and tell the people that he has been raised from the dead. This last deception will be worse than the first.”