Hello,
From what I understand, reincarnation was never a Christian belief. It is Buddhist. The Buddhist usually cremate their dead because to them the soul is more important than the body. They believe that the soul or spirit of a person will find itself in another body. Christians, on the other hand, bury their dead because the body is just as important as the soul. The body and soul are one just as the Father and the Son are one despite that the Son has a human nature.
When Christ died and was resurrected, His resurrection was both body and soul. He resurrected in a glorified body. On Christ's second coming, our bodies will also be resurrected. It will change into a glorified body. The spiritual beings that existed in Heaven were the angels. And angels were never created in the image of God. Only man was created in His image. Everything else is a reflection of God's goodness. Angels can appear in human form, but in essence, they are NOT human. When God said that He knew us before we existed, that is because He is the All-knowing God who created us. He also knows those who will be born 10 years from now. He knows everything and all future events.
As for the early Christians, they never supported reincarnation and there are many writings showing this. New agers often say that Origen supported reincarnation, but they give no evidence showing it. Origen was, in fact, against reincarnation. Below is what Origen stated in AD 229:
From Origen: "[Scripture says] ‘And they asked him, "What then? Are you Elijah?" and he said, "I am not"’ [John 1:21]. No one can fail to remember in this connection what Jesus says of John: ‘If you will receive it, this is Elijah, who is to come’ [Matt. 11:14]. How then does John come to say to those who ask him, ‘Are you Elijah?’—‘I am not’? . . . One might say that John did not know that he was Elijah. This will be the explanation of those who find in our passage a support for their doctrine of reincarnation, as if the soul clothed itself in a fresh body and did not quite remember its former lives. . . . [H]owever, a churchman, who repudiates the doctrine of reincarnation as a false one and does not admit that the soul of John was ever Elijah, may appeal to the above-quoted words of the angel, and point out that it is not the soul of Elijah that is spoken of at John’s birth, but the spirit and power of Elijah" (Commentary on John 6:7 [A.D. 229]).
"As for the spirits of the prophets, these are given to them by God and are spoken of as being in a manner their property [slaves], as ‘The spirits of the prophets are subject to the prophets’ [1 Cor. 14:32] and ‘The spirit of Elijah rested upon Elisha’ [2 Kgs. 2:15]. Thus, it is said, there is nothing absurd in supposing that John, ‘in the spirit and power of Elijah,’ turned the hearts of the fathers to the children and that it was on account of this spirit that he was called ‘Elijah who is to come’" (ibid.).
"If the doctrine [of reincarnation] was widely current, ought not John to have hesitated to pronounce upon it, lest his soul had actually been in Elijah? And here our churchman will appeal to history, and will bid his antagonists [to] ask experts of the secret doctrines of the Hebrews if they do really entertain such a belief. For if it should appear that they do not, then the argument based on that supposition is shown to be quite baseless" (ibid.).
"Someone might say, however, that Herod and some of those of the people held the false dogma of the transmigration of souls into bodies, in consequence of which they thought that the former John had appeared again by a fresh birth, and had come from the dead into life as Jesus. But the time between the birth of John and the birth of Jesus, which was not more than six months, does not permit this false opinion to be considered credible. And perhaps rather some such idea as this was in the mind of Herod, that the powers which worked in John had passed over to Jesus, in consequence of which he was thought by the people to be John the Baptist. And one might use the following line of argument: Just as because the spirit and the power of Elijah, and not because of his soul, it is said about John, ‘This is Elijah who is to come’ [Matt. 11:14] . . . so Herod thought that the powers in John’s case worked in him works of baptism and teaching—for John did not do one miracle [John 10:41]—but in Jesus [they worked] miraculous portents" (Commentary on Matthew 10:20 [A.D. 248]).
"Now the Canaanite woman, having come, worshipped Jesus as God, saying, ‘Lord, help me,’ but he answered and said, ‘It is not possible to take the children’s bread and cast it to the little dogs.’ . . . [O]thers, then, who are strangers to the doctrine of the Church, assume that souls pass from the bodies of men into the bodies of dogs, according to their varying degree of wickedness; but we . . . do not find this at all in the divine Scripture" (ibid., 11:17).
"In this place [when Jesus said Elijah was come and referred to John the Baptist] it does not appear to me that by Elijah the soul is spoken of, lest I fall into the doctrine of transmigration, which is foreign to the Church of God, and not handed down by the apostles, nor anywhere set forth in the scriptures" (ibid., 13:1).