That's a tough question, I'll give you the short version of my answer. LOL
Jesus said HE is the Light of the World. And before Abraham was, "I AM".
1Peter 1:
18 Forasmuch as ye know that ye were not redeemed with corruptible things,
as silver and gold, from your vain conversation
received by tradition from your fathers;
19 But with the precious blood of Christ, as of a lamb without blemish and without spot:
20 Who verily was foreordained
before the foundation of the world, but
was manifest in these last times for you,
Gen. 1:
3 And God said, Let there be light: and there was light.
So it seems God so loved the World, that He sent His Son, the Word of God, in which all things were created, from the very beginning.
And the people of God, like Abraham, knew about Him becoming this Lamb.
Gen. 22:
8 And Abraham said, My son, God will
provide himself a lamb for a burnt offering: so they went both of them together.
And because of transgressions, this same Son created a Priesthood,
given to the Levites, which contained sacrificial "Works/deeds of the Law" for the cleansing of sin. This sacrifice foreshadowed a sacrificial event in which this same Son would come to Earth as a man.
Ex. 12:
5 Your lamb shall be without blemish, a male of the first year:
ye shall take it out from the sheep, or from the goats:
6 And ye shall keep it up until the fourteenth day of the same month: and the whole assembly of the congregation of Israel shall kill it in the evening.
Duet. 18:
15 The LORD thy God will raise up unto thee a Prophet
from the midst of thee,
of thy brethren,
like unto me; unto him ye shall hearken;
And this same Son foretold of this event several times and what it would mean. Jeremiah 31 is one such place.
Jer. 31:
31 Behold,
the days come, saith the LORD, that I will make a new covenant with the house of Israel, and with the house of Judah:
32 Not according to the covenant that I made with their fathers in the day
that I took them by the hand to bring them out of the land of Egypt ( Levites to administer His Instructions, and to perform sacrificial "works/deeds of the Law for atonement) which my covenant they brake, although I was an husband unto them, saith the LORD (For finding fault with THEM)
33 But this
shall be the covenant that I will make with the house of Israel;
After those days, saith the LORD,
I will put my law in their inward parts, and write it in their hearts; and will be their God, and they shall be my people.
No more Levite Priests to administer His Laws!!!
34 And they shall teach no more every man his neighbour, and every man his brother, saying, Know the LORD: for they shall all know me, from the least of them unto the greatest of them, saith the LORD: for
I will forgive their iniquity, and I will remember their sin no more.
No more Levite Priesthood "works/deeds of the Law" for remission of sins!!!
In Galatians the Levite Priests were still promoting their version of this Priesthood that was "ADDED" 430 years after Abraham.
They didn't believe that Son, the Word of God. So they didn't believe the Son, the Word of God when He became a man like Moses.
John 5:
46 For had ye believed Moses, ye would have believed me: for he wrote of me.
47 But if ye believe not his writings, how shall ye believe my words?
So it seems then that this "ADDED" Law was to be in place until the "SEED" should come. This "ADDED" Law being the Priesthood Abraham didn't have. (Levi wasn't born yet)
I used to think it was when Jesus became of age to preach. A two year old can't offer his life for or teach anyone. I noticed that Jesus forgave sins without ever shedding a drop of goats or Lambs blood as the "added" Law prescribed. But then I thought about all the sins of His people that were forgiven by His Blood before He gave those "works of the Law" to Levi, like Abraham. So all the sins of the world which have been forgiven, was because of His Sacrifice and His Blood that He gave for them, "but was manifest in these last days for us".
Heb. 7:26 For such an high priest became us,
who is holy, harmless, undefiled,
separate from sinners,
and made higher than the heavens;
27 Who needeth not daily, as those high priests, to offer up sacrifice, first for his own sins, and then for the people's:
for this he did once, when he offered up himself.
28 For the (Added Priesthood) law
maketh men high priests which have infirmity; but the word of the oath, which was since (Before) the "ADDED" law,
maketh the Son, who is consecrated for evermore.
So I would say His Death and Resurrection, when God the Father, raised His Son, the Word of God which became Flesh, from the Dead.
When He said "It is finished".
This was when the Man Jesus became our High Priest. This is the end of the "ADDED" Priesthood "works/deeds of the Law" for remission of sins.
I tried to keep it short