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 Isaiah 7:14Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel.
American King James Version×, "'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 1:23Behold, a virgin shall be with child, and shall bring forth a son, and they shall call his name Emmanuel, which being interpreted is, God with us.
American King James Version×). 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 John 1:1-14  In the beginning was the Word, and the Word was with God, and the Word was God.
 The same was in the beginning with God.
 All things were made by him; and without him was not any thing made that was made.
 In him was life; and the life was the light of men.
 And the light shines in darkness; and the darkness comprehended it not.
 There was a man sent from God, whose name was John.
 The same came for a witness, to bear witness of the Light, that all men through him might believe.
 He was not that Light, but was sent to bear witness of that Light.
 That was the true Light, which lights every man that comes into the world.
 He was in the world, and the world was made by him, and the world knew him not.
 He came to his own, and his own received him not.
 But as many as received him, to them gave he power to become the sons of God, even to them that believe on his name:
 Which were born, not of blood, nor of the will of the flesh, nor of the will of man, but of God.
 And the Word was made flesh, and dwelled among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth.
American King James Version×).
Some of the disciples called Jesus God directly. When Thomas saw His wounds, he exclaimed, "My Lord and my God!" (John 20:28 John 20:28And Thomas answered and said to him, My LORD and my God.
American King James Version×). 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 Titus 2:13Looking for that blessed hope, and the glorious appearing of the great God and our Savior Jesus Christ;
American King James Version×as "our great God and Savior Jesus Christ." Peter likewise calls Him "our God and Savior Jesus Christ" (2 Peter 1:1 2 Peter 1:1Simon Peter, a servant and an apostle of Jesus Christ, to them that have obtained like precious faith with us through the righteousness of God and our Savior Jesus Christ:
American King James Version×).
The book of Hebrews is most emphatic that Jesus is God. Hebrews 1:8 Hebrews 1:8But to the Son he said, Your throne, O God, is for ever and ever: a scepter of righteousness is the scepter of your kingdom.
American King James Version×, applying Psalms 45:6 Psalms 45:6Your throne, O God, is for ever and ever: the scepter of your kingdom is a right scepter.
American King James Version×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 Hebrews 1:4-13  Being made so much better than the angels, as he has by inheritance obtained a more excellent name than they.  For to which of the angels said he at any time, You are my Son, this day have I begotten you? And again, I will be to him a Father, and he shall be to me a Son?  And again, when he brings in the first-begotten into the world, he said, And let all the angels of God worship him.  And of the angels he said, Who makes his angels spirits, and his ministers a flame of fire.  But to the Son he said, Your throne, O God, is for ever and ever: a scepter of righteousness is the scepter of your kingdom.  You have loved righteousness, and hated iniquity; therefore God, even your God, has anointed you with the oil of gladness above your fellows.  And, You, Lord, in the beginning have laid the foundation of the earth; and the heavens are the works of your hands:  They shall perish; but you remain; and they all shall wax old as does a garment;  And as a clothing shall you fold them up, and they shall be changed: but you are the same, and your years shall not fail.  But to which of the angels said he at any time, Sit on my right hand, until I make your enemies your footstool?
American King James Version×), superior to Moses (Hebrews 3:1-6 Hebrews 3:1-6  Why, holy brothers, partakers of the heavenly calling, consider the Apostle and High Priest of our profession, Christ Jesus;  Who was faithful to him that appointed him, as also Moses was faithful in all his house.  For this man was counted worthy of more glory than Moses, inasmuch as he who has built the house has more honor than the house.  For every house is built by some man; but he that built all things is God.  And Moses truly was faithful in all his house, as a servant, for a testimony of those things which were to be spoken after;  But Christ as a son over his own house; whose house are we, if we hold fast the confidence and the rejoicing of the hope firm to the end.
American King James Version×) and greater than the high priests (Hebrews 4:14-16 Hebrews 4:14-16  Seeing then that we have a great high priest, that is passed into the heavens, Jesus the Son of God, let us hold fast our profession.  For we have not an high priest which cannot be touched with the feeling of our infirmities; but was in all points tempted like as we are, yet without sin.  Let us therefore come boldly to the throne of grace, that we may obtain mercy, and find grace to help in time of need.
American King James Version×; Hebrews 5:1-10 Hebrews 5:1-10  For every high priest taken from among men is ordained for men in things pertaining to God, that he may offer both gifts and sacrifices for sins:  Who can have compassion on the ignorant, and on them that are out of the way; for that he himself also is compassed with infirmity.  And by reason hereof he ought, as for the people, so also for himself, to offer for sins.  And no man takes this honor to himself, but he that is called of God, as was Aaron.  So also Christ glorified not himself to be made an high priest; but he that said to him, You are my Son, to day have I begotten you.  As he said also in another place, You are a priest for ever after the order of Melchisedec.  Who in the days of his flesh, when he had offered up prayers and supplications with strong crying and tears to him that was able to save him from death, and was heard in that he feared;  Though he were a Son, yet learned he obedience by the things which he suffered;  And being made perfect, he became the author of eternal salvation to all them that obey him;  Called of God an high priest after the order of Melchisedec.
American King James Version×). He is greater than all these because He is God—along with the Father.