The curtain torn...

 36  One man ran, filled a sponge with wine vinegar, put it on a stick, and offered it to Jesus to drink. "Now leave him alone. Let's see if Elijah comes to take him down," he said.  37 With a loud cry, Jesus breathed his last.  38 The curtain of the temple was torn in two from top to bottom. 39 And when the centurion, who stood there in front of Jesus, heard his cry and saw how he died, he said, "Surely this man was the Son of God!"  Mark 15:36-39

Why was the curtain torn in two?  Some believe it was the curtain inside the temple that separated the Holy of Holies from the rest of the temple.  Only the High Priest could enter the Holy of Holies, and then only enter it once a year to offer a sacrifice to God for the sins of the people.  When Jesus died, the curtain was no longer needed, as the way to God was now through Jesus.

Others compare it to the verse below, when Jesus was baptized:

9 At that time Jesus came from Nazareth in Galilee and was baptized by John in the Jordan. 10 As Jesus was coming up out of the water, he saw heaven being torn open and the Spirit descending on him like a dove. 11 And a voice came from heaven: "You are my Son, whom I love; with you I am well pleased."  Mark 1:9-11

There were two curtains in the temple, one at the entrance of the temple, the outer veil, and one separating the Holy of Holies from the rest of the temple, the inner veil. The curtain at the entrance of the temple was said to be a Babylonian tapestry with a pictorial of the heavens on it.  And because the centurion saw Jesus die and and then he saw the curtain tear, that it was the outer veil that he had to see.  Mark may have compared the two, baptism and death, as the bookends of Jesus' ministry on earth.  Or, he may have thought of both instances as respective beginnings for Jesus.  The opening of the heavens at his baptism as the beginning of his earthly ministry, and the tearing of the temple curtain as the beginning of his real reason, to save those who believe in Him as Savior. The centurion may have been the first saved.