Up until 1583 AD, the time when the Gregorian calendar was introduced, the 25th Kislev and 25th December were the same day.
This is not a true statement, BTW.
What evidence do you have supporting the 4-6 time line for the birth, I see none?
I've given quite a bit on another thread, but I will cut and paste relevant parts here.
To start off, we know that the year is wrong. A simple clerical error threw the year off by quite a bit. In 525 AD, Pope John I commissioned the scholar Dionysius Exiguus to establish a feast calendar for the Church. Dionysius also estimated the year of Christ's birth based upon the founding of the city of Rome. Because of insufficient historical data he arrived at a date at least a few years later than the actual event.
Since the Bible tells us that Jesus was born before the death of Herod, and we know that Herod died in 4 BC by our current reckoning, then Jesus had to have been born before that. (Although it is possible that he died as late as April in 3 BC.) We know from other Scriptural evidence that it was as much as two years before the arrival of the Magi. Based on the information we have on hand, then Jesus was born somewhere between 4-6 BC. Most evidence points to 6 BC, and 25th of September would be the first day of the Feast of Tabernacles in that year. (If you want, I will repost the Scriptural evidence that he was born at this time of year.) The latest he could have been born would be 3 BC, and the earliest would be 8 BC, but 6 BC fits all the Scriptural prophecies.
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Jesus made many prophecies concerning his death, burial, and resurrection. Matthew 16:21 says, “From that time forth began Jesus to shew unto his disciples, how that he must go unto Jerusalem, and suffer many things of the elders and chief priests and scribes, and be killed, and be raised again the third day.” Matthew 17:22-23 says, “And while they abode in Galilee, Jesus said unto them, The Son of man shall be betrayed into the hands of men: And they shall kill him, and the third day he shall be raised again. And they were exceeding sorry.”
[Mark 8:31] Here, Jesus states explicitly that His resurrection would occur after or at the end of three days. Not after one day, or one and a half days, or two days, but after three days. So, Jesus must have been resurrected just as the third day was ending, which was three days and three nights after He was placed in the tomb.
This is the only way that the prophecies could be consistent. Are they consistent? If not, the Bible is worthless, because Jesus is a flawed Messiah. But, the prophecies are consistent. When some of His followers came to the tomb early on Sunday morning, the angels told them that He was no longer there. Matthew 28:6 tells us, “He is not here: for he is risen, as he said. Come, see the place where the Lord lay.”
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We know that 29AD is the only year that fulfills all the prophecies for his death, burial, and resurrection. Forget the secular evidence; if this event does not fulfill prophecy, then the Scriptures are useless and should be thrown away.
Jesus was crucified on Wednesday, and entombed on Thursday, according to the Jewish calendar (Wednesday evening by our reckoning). That Thursday was a high Sabbath; it was the Passover. Christ is our Passover and He shields us, just as the little Passover lambs shielded the firstborn children in Egypt. Remember, the Passover is just for the firstborn, not all the children. It’s about position. Jesus died for all, but the Bible tells us that His blood was shed for many. He died that all might be saved. We’re saved by His death, but we’re sanctified by His blood.
The second day in the tomb, beginning Thursday evening, was a high Sabbath; it was the first day of the Feast of Unleavened Bread. This feast represents cleanness; you had to sweep all the leaven or impurities from your house. 1 Corinthians 5:6-7 says, “Your glorying is not good. Know ye not that a little leaven leaveneth the whole lump? Purge out therefore the old leaven, that ye may be a new lump, as ye are unleavened. For even Christ our passover is sacrificed for us,” and Galatians 5:9 says, “A little leaven leaveneth the whole lump.” Leaven represents Phariseeism, or the traditions of men, as opposed to the laws of God. Sort of like the traditions surrounding Easter.
The third day in the grave, from Friday evening until Saturday evening, was a weekly Sabbath, which was given as a covenant to man, as a type of the coming Kingdom rest of our Lord and Savior Jesus Christ.
He rested in the heart of the Earth for three Sabbaths, and that completely and utterly fulfilled the sign of His authenticity, that He would spend three days and three nights in the heart of the earth. The Bible tells us that when He died, his body was in the grave, His spirit went to be with the father, and His soul went to hell (Sheol). In all three He rested, and all three are represented in the three types of Sabbaths that are represented here. It’s complete: Body, soul, and spirit.
This could only have happened in 29AD. Subtract 33 years from that, and you get 4 BC as the latest possible date for his birth and 6 BC as the earliest. Anything else would leave prophecy unfulfilled and he would be a flawed Messiah.
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Not only are these the only dates that could fulfill prophecy and history, but they are also the only dates that fulfill the implications of the Feasts.
It does not, however, fulfill the wishes and desires of modern man.
I don't think it's important when we celebrate. But, I do think it's important to know when and know the full implications in Scripture.