Today we observe the day that Christ was crucified for our transgressions:

Matthew 27:45 Now from the sixth hour there was darkness over all the land until the ninth hour. 46 And about the ninth hour Jesus cried out with a loud voice, saying, “Eli, Eli, lema sabachthani?” that is, “My God, my God, why have you forsaken me?” 47 And some of the bystanders, hearing it, said, “This man is calling Elijah.” 48 And one of them at once ran and took a sponge, filled it with sour wine, and put it on a reed and gave it to him to drink. 49 But the others said, “Wait, let us see whether Elijah will come to save him.” 50 And Jesus cried out again with a loud voice and yielded up his spirit. 51 And behold, the curtain of the temple was torn in two, from top to bottom. And the earth shook, and the rocks were split. 52 The tombs also were opened. And many bodies of the saints who had fallen asleep were raised, 53 and coming out of the tombs after his resurrection they went into the holy city and appeared to many. 54 When the centurion and those who were with him, keeping watch over Jesus, saw the earthquake and what took place, they were filled with awe and said, “Truly this was the Son of God!”

Amazingly we read about the account in Isaiah:

Isaiah 53:3 He was despised and rejected by men; a man of sorrows, and acquainted with grief; and as one from whom men hide their faces he was despised, and we esteemed him not. 4 Surely he has borne our griefs and carried our sorrows; yet we esteemed him stricken, smitten by God, and afflicted. 5 But he was wounded for our transgressions; he was crushed for our iniquities; upon him was the chastisement that brought us peace, and with his stripes we are healed. 6 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. 7 He was oppressed, and he was afflicted, yet he opened not his mouth; like a lamb that is led to the slaughter, and like a sheep that before its shearers is silent, so he opened not his mouth. 8 By oppression and judgment he was taken away; and as for his generation, who considered that he was cut off out of the land of the living, stricken for the transgression of my people? 9 And they made his grave with the wicked and with a rich man in his death, although he had done no violence, and there was no deceit in his mouth. 10 Yet it was the will of the LORD to crush him; he has put him to grief; when his soul makes an offering for guilt, he shall see his offspring; he shall prolong his days; the will of the LORD shall prosper in his hand. 11 Out of the anguish of his soul he shall see and be satisfied; by his knowledge shall the righteous one, my servant, make many to be accounted righteous, and he shall bear their iniquities. 12 Therefore I will divide him a portion with the many, and he shall divide the spoil with the strong, because he poured out his soul to death and was numbered with the transgressors; yet he bore the sin of many, and makes intercession for the transgressors.

Why do Christians in the 21st century believe that a man was crucified on a cross almost 2,000 years ago and that he rose again from the dead? Because of the witness of Scripture. Isaiah said that it would happen and it did.