Preaching Gospel of Thomas saying 50

49 Jesus said: Blessed are the solitary and the elect, for you will find the kingdom, for you came forth from it, (and) you will return to it again. 50 Jesus said, "If they say to you (plur.), 'Where are you from?' say to them, 'It is from light that we have come - from the place where light, of its own accord alone, came into existence and [stood at rest]. And it has been shown forth in their image.' If they say to you, 'Is it you?' say 'We are its offspring, and we are the chosen of the living father.' If they ask you, 'What is the sign of your father within you?' say to them, 'It is movement and repose.'"

This saying should by read with saying 49

Here Jesus is teaching his disciples about what to say if they are ask questions on missionary work. 

But Who are “they”? they are people outside of the community who are asking the missionaries who they are, so Jesus is telling them how to answer their questions. Or they could be the Pharisees or leaders of synagogues in Gaille or Juedea. 

“say to them, we came from the light” This part of the saying continues the thought of Saying 24. The subject-matter of this saying is much the same as that of Saying 49; for the place of light cf. Sayings 24, 77. Jesus is speaking about himself here as the light of the world and The disciples are the light of the world (Matthew 7:14) because Jesus is the light of the world (John 8:12). 

They are from above, from the place where the light shines in the darkness (John 1:5). They are sons of the light (saying 24), and the elect. If men ask for a sign, as they asked Jesus (Mark 8:11-12; Matthew 16:1-4; Luke 11:16, 29-30), no startling miracle can be shown them, but only 'a movement and a rest.' The 'rest' must be the rest characteristic of the kingdom (Sayings 1 [Greek], 52, 90); the 'movement' is
for the place of light cf. Sayings 24, 77. 

God is light and he is the pre-existent one for there was never a time when God did not exist and Theos was the logos that is deity was the plan and in this plan was life and the life was the light of men. that is spiritual life, which is produced by the light of God's word -- not natural life! And thereby setting the parameters of "beginning" and "creation": the "beginning" of God's revelation of His Son to the world, and the new, spiritual "creation" of believers in him. The word (plan) became flesh at Jesus’ baptism and the text of the Diatessaron confirms this see section III. 46: 

This man came to bear witness, that he might bear witness to the light, that every man might believe through his mediation. He was not the light, bat that he might bear witness to the light, which was the light of truth, that giveth light to every man coming into the world. He was in the world, and the world was made by him, and the world knew him not. He came unto his own, and his own received him not. And those who received him, to them gave he the power that they might be sons of God,--those which believe in his name: which were born, not of blood, nor of the will of the flesh, nor of the will of a man, but of God. And the Word became flesh, and took up his abode among us; and we saw his glory as the glory of the only Son from the Father, which is full of grace and equity. John bare witness of him, and cried, and said, This is he that I said cometh after me and was before me, because he was before me. And of his fullness received we all grace for grace. For the law was given through the mediation of Moses, but truth and grace were through Jesus Christ. (Matthew 3:16 

15 But Jesus answered and said to him, "Permit it to be so now, for thus it is fitting for us to fulfil all righteousness." Then he allowed him. 
16 When Jesus was immersed a bright light played upon the water, so that all who had come there were afraid. After his immersion, Jesus came up immediately from the water; and behold, the heavens were opened to him, and he saw the spirit of God descending like a dove and alighting upon him. 
17 And suddenly a voice came from heaven, saying, "This is my beloved Son, in whom I am well pleased." In Luke 3:22 Instead of “You are my one dear Son; in you I take great delight,” one Greek ms and several Latin mss and church fathers (D it Ju [Cl] Meth Hil Aug) quote Ps 2:7 outright with “You are my Son; today I have begotten you.” 

here than the place where the light came into being was at Jesus’ baptism when the word was made flesh although the became flesh at his birth however at his baptism the word truly became flesh when Jesus was anointed with the holy spirit he became the Messiah the promised seed of the woman and son of God and after his resurrection Jesus was born once more 
Rom 1:4 And declared to be the Son of God with power, according to the spirit of holiness, by the resurrection from the dead: so Jesus was born 3 times the 1st by the holy spirit and Mary and this made him son of man and son of God and the second at his baptism and the 3rd after his resurrection so we have light at his baptism and at the transfiguring and after his resurrection from the dead he became light and this is the place where light came into being by its self in the Lord Jesus 
Mt 4:16 Mt 17:2Lu 1:79Lu 11:35 Lu 11:36 Lu 16:8 Ac 9:3 Ac 12:7 Ac 22:6 Ac 22:9Ac 22:11 Ac 26:13 Ac 26:18 Eph 5:8 Col 1:12 1Th 5:5 1Ti 6:16 1Jo 1:5 1Jo 1:7 1Jo 1Jo 2:9 1Jo 2:10 Re 21:11 Re 21:23 Re 21:24 Re 22:5 

The relationship to the living Father has been mentioned in Saying 3 (cf. Saying 37). 
For the 'image' the light dwells in our image the image is a reference to being the twin bother of Jesus for our image should be the same as the son of God and being the same as the Son of God we are sons of God see Saying 83. 

The 'rest' must be the rest characteristic of the kingdom (Sayings 1 [Greek], 52, 90);
The 'movement' may 

50) Jesus said, "If they [religious leaders] say to you [the Elect], 'Where did you come from?', say to them, 'We came from the light [we were born of holy spirit], the place where the light came into being on its own accord ["the holy spirit blows where it will and we do not know it" - i.e. we have no part in the calling that takes place among the Elect] and established [itself] and became manifest through their image [their forms of worship, in their ordinances, traditions and festivals etc.. all lower/outward {fleshly} forms and rituals) which were designed to point them to the advent of Messiah].' If they say to you, 'Is it [the light] you?', say, 'We are its children [born again in spirit], we are the elect [sons] of the Living Father.' If they ask you, 'What is the sign of your father in you [how can we believe you? Show us proof]?', say to them, 'It is movement [being drawn out of the world and into the Kingdom - i.e. from darkness to light, from the letter to the spirit, from unrighteousness to righteousness, etc..] and repose [true rest in Him knowing He is working a work in you - i..e you "cease from your own works].'" (Mt 7:14; Jn 8:12; Jn 1:5; Mk 8:11-12; Mt 16:1-4; Lk 11:16, 29-30; compare Sayings 84; 83; 24; 77; 3; 37; 51; 52; 53)

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