Jesus called Himself the Son of man while He was also the Son of God because:
Jesus was literally the Son of man, both in a purely historic sense ( Luke 1:31–35; Rom. 1:3, 4; Gal. 4:4) and in a higher sense. The title, Son of man, designates Him as the incarnate Christ (John 1:14; Phil. 2:6–8). It points to the miracle whereby the Creator and creature were united in one divine-human person. It testifies to the truth that the sons of men may indeed become sons of God (John 1:12; Gal. 4:3–7; 1 John 3:1, 2). Deity was identified with humanity in order that humanity might be transformed into the image of divinity.
B-All three synoptic writers use the term Son of man (Matt. 9:6; Mark 2:10; Luke 5:24). It was Christ’s favorite designation for Himself, and appears in the gospels some 80 times. No one, however, addressed Him by this title, nor do any of the gospel writers refer to Him by it. This title was understood at least among some Jews as a name for the Messianic ruler of the new kingdom to be established.
Except under oath (Matt. 26:63, 64; Mark 14:61, 62), and in private to those ready to believe in Him as the Christ (Matt. 16:16, 17; John 3:13–16), Jesus made no direct Messianic claims. It was His purpose that men should recognize in His life, His words, and His works, evidence that the prophecies of the Messiah had met their fulfillment in Him.
The term Son of man is used in Scripture in contexts of Christ’s deity. For example, the Bible says that only God can forgive sins (Isaiah 43:25; Mark 2:7). But as the Son of man, Jesus had the power to forgive sins (Mark 2:10). Another Example shows that Christ will return to Earth as the Son of man in clouds of glory to reign on Earth (Matthew 26:63-64). In this passage, Jesus is citing Daniel 7:13 where the Messiah is described as the “Ancient of Days,” a phrase used to indicate His deity (Daniel 7:9).
C-When the high priest asked Jesus if He was the Son of God (Matthew 26:63), Jesus answered confirming that He was the Son of man who would come in power and great glory (verse 64). This indicated that Jesus Himself used the phrase Son of man to indicate His deity as the Son of God.
In His service,