His disciples were monotheistic Jews. For them to agree that Jesus was God, and then to give their lives for this belief, tells us that they had come to see for themselves that the claims Jesus made about Himself were so convincing as to leave no doubt in their minds.
The first Gospel writer, Matthew, opens with the story of the virgin birth of Jesus. Matthew comments on this miraculous event with the quote from Isaiah 7:14, "'Behold, the virgin shall be with child, and bear a Son, and they shall call His name Immanuel,' which is translated, 'God with us'" (Matthew 1:23). Matthew is making it clear that he understands that this child is God—"God with us."
John is likewise explicit in the prologue to his Gospel: "In the beginning was the Word, and the Word was with God, and the Word was God . . . And the Word became flesh and dwelt among us" (John 1:1-14).
Some of the disciples called Jesus God directly. When Thomas saw His wounds, he exclaimed, "My Lord and my God!" (John 20:28). Some view this as simply an expression of surprise. But such profane use of God's name would have been unacceptable among the Jews of that day. Paul refers to Jesus in Titus 2:13 as "our great God and Savior Jesus Christ." Peter likewise calls Him "our God and Savior Jesus Christ" (2 Peter 1:1).
The book of Hebrews is most emphatic that Jesus is God. Hebrews 1:8, applying Psalm 45:6 to Jesus Christ, states: "But to the Son He [the Father] says: 'Your throne, O God, is forever and ever.'" Other parts of Hebrews explain that Jesus is higher than the angels (Hebrews 1:4-13), superior to Moses (Hebrews 3:1-6) and greater than the high priests (Hebrews 4:14-16; Hebrews 5:1-10). He is greater than all these because He is God—along with the Father.