John 12 in New World Translation:
Although he had performed so many signs before them, they were not putting faith in him, so that the word of Isaiah the prophet might be fulfilled, ... The reason why they were not able to believe is that again Isaiah said: “He has blinded their eyes and has made their hearts hard, so that they would not see with their eyes and understand with their hearts and turn around and I heal them.”Isaiah said these things because he saw his glory, and he spoke about him.
All the same, many even of the rulers actually put faith in him, but they would not acknowledge him because of the Pharisees, so that they would not be expelled from the synagogue;for they loved the glory of men even more than the glory of God. - NWT
Isaiah saw God in heaven in Isaiah Ch. 6. The Apostle John says Isaiah saw Jesus. He equates the disbelieving of God in Isaiah, to the unbelieving Jews in John 12.
Also:
Take heed therefore unto yourselves, and to all the flock, over the which the Holy Ghost hath made you overseers, to feed the church of God, which he hath purchased with his own blood. Acts: 20: 28 (KJV)
The NWT was forced to add "Son" to change the meaning of this scripture:
"Pay attention to yourselves and to all the flock, among which the holy spirit has appointed you overseers, to shepherd to congregation of God, which he purchased with the blood of his own [Son]." Acts 20;24 (NWT 1984 edition)
The brackets in the NWT indicate words that are not in the original text.The 2013 NWT edition eliminates the parenthesis, but leaves "Son" anyway.
Conclusion: The earliest Christians understood that Jehovah (God) took on human flesh in Jesus, the son of Mary, because they plainly wrote about it.
Reading Hebrews 1 & Revelation 1 in most anything but a NWT, also makes this crystal clear.