As Muslims we know that Jesus was not crucified on the cross, but a likeness of him instead. How were the people present there supposed to know that it was not Jesus? They would've assumed that it was Jesus.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

The disciples of 'Eesaa (Jesus) knew that the one who was crucified was not him. Adh-Dhahhak said: 'The story was that when the Jews wanted to kill 'Eesaa he entered to the disciples and there were twelve of them and he asked them: "Who amongst you would go out and be killed and he will be with me in Paradise?' The narrator related the whole story. The same story was narrated by Ibn 'Abbaas so the disciples who were with 'Eesaa knew for certain that he was not killed and that he was taken up to the heavens. Even those who killed the person on the cross doubted that it was the Messiah. Allaah says (which means): {For surely; they killed him not [i.e. 'Eesaa (Jesus), son of Maryam (Mary).}[4:157]. Ibn Katheer interpreted the verse to mean that they were not certain that they killed 'Eesaa but they doubted about him. He interpreted the following verse: {…but they killed him not, nor crucified him, but the resemblance of 'Eesaa (Jesus) was put over another man (and they killed that man).}[4:157]. He said: 'The disciples who were inside the house saw 'Eesaa when he was lifted. Apart from them, all other people thought the same way as the Jews thought, that the one who was crucified was the Messiah, son of Maryam (Mary), may Allaah exalt their mention. They also mentioned that Maryam, may Allaah exalt her mention, sat down under the person who was crucified and wept, Allaah knows best. All this, is a test from Allaah to his slaves, as in this there is a great wisdom.'

The conclusion from all of this is that not all the people who were at that time believed that the Messiah was crucified. Saying that they must have believed that it was the Messiah who was crucified is not an absolute statement. Anyway, whoever the following verse has reached: "…but they killed him not, nor crucified him, but the resemblance of 'Eesaa (Jesus) was put over another man (and they killed that man)", and still believes that the person who was crucified was the Messiah, is a non-Muslim and a disbeliever in Allaah as he is belying the Qur'an.

Allaah knows best.