Mark wrote the Gospel of Mark to bear witness that Jesus was the Christ, prophesied in the Old Testament and the Son of God. Everything in Mark's Gospel is actually directed at this topic. Mark 1:2-3, 8,11, Psalm 2:7, Isaiah 42:1
Mark first decided on the subject of the Gospel of Mark and wrote the Gospel of Mark. In other words, Jesus is the Christ and the Son of God. (Mark 1:1)
Mark 1:1 The beginning of the gospel of Jesus Christ, the Son of God;
Also, Mark reveals that John the Baptist was prophesied in the Old Testament to send someone who would prepare the way for Christ. (Mark 1:2-3,8)
Mark 1:2 As it is written in the prophets, Behold, I send my messenger before thy face, which shall prepare thy way before thee. 3 The voice of one crying in the wilderness, Prepare ye the way of the Lord, make his paths straight. 8 I indeed have baptized you with water: but he shall baptize you with the Holy Ghost.
In the Old Testament it was foretold that Christ would be poured out with the Holy Spirit and would be called the Son of God. (Isaiah 42:1, Psalm 2:7)
Isaiah 42:1 Behold my servant, whom I uphold; mine elect, in whom my soul delighteth; I have put my spirit upon him: he shall bring forth judgment to the Gentiles.
Psalm 2:7 I will declare the decree: the Lord hath said unto me, Thou art my Son; this day have I begotten thee.
When John the Baptist baptized Jesus, the Holy Spirit came upon Jesus, and God called Jesus the Son. That is, Jesus is the Christ prophesied in the Old Testament. (Mark 1:9-11)
Mark 1:9 And it came to pass in those days, that Jesus came from Nazareth of Galilee, and was baptized of John in Jordan. 10 And straightway coming up out of the water, he saw the heavens opened, and the Spirit like a dove descending upon him: 11 And there came a voice from heaven, saying, Thou art my beloved Son, in whom I am well pleased.