Why was the cross necessary?
Sin is a big deal & carries a big MUST BE PAID price.
Rom 6:23 the wage (cost/price) for sin is death.
The 1st animal sacrifice for sin happen in the garden of Eden. Where Adam & Eve CHOOSE to SIN/transgress heavenly law. Their sin/transgression had to be paid for.
God could execute sins MUST/JUST penalty "DEATH" onto them. He choose to make atonement/pay sins required wage: BLOOD & DEATH via an animal sacrifice.
Gen 3:21 Unto Adam also and to his wife did the LORD God make coats of skins, and clothed them.
(NOTE: Animals died to cover/clothe/pay sins just/must required wage/pentley "DEATH")
Animal sacrifices (innocent dying in place guilty) contuinue for millenia. Until the ultimate "Once & For All" innoncent sinless sacrifice is offered. The Eternal Son of God, Jesus the Christ offers Himself & pays for Adams caused crisis!
BTW, animal sacrifices only covered for 1 year & couldn't remove sin.
Cain & Abel
Gen 4:4 And Abel, he also brought of the firstlings of his flock and of the fat thereof. And the LORD had respect unto Abel and to his offering:
Noah
Gen 8:20 And Noah builded an altar unto the LORD; and took of every clean beast, and of every clean fowl, and offered burnt offerings on the altar.
Abraham
Gen 22:13 And Abraham lifted up his eyes, and looked, and behold behind him a ram caught in a thicket by his horns: and Abraham went and took the ram, and offered him up for a burnt offering in the stead of his son.
Covenant at Sinai
And there were many animal sacrifices (millions?) offered for sin under the Covenant of Sinai. Until the ultimate "Once & For All" innocent sinless sacrifice is offered, the Eternal Son of God, Jesus the Christ! Christ's willful, sinless sin offering, when appropriated/seized, doesn't only cover, it TAKES AWAY/REMOVES sin:
John 1:29 The next day John seeth Jesus coming unto him, and saith, Behold the Lamb of God, "which taketh away the sin of the world".
Rev 1:5 And from Jesus Christ, who is the faithful witness, and the first begotten of the dead, and the prince of the kings of the earth. Unto him that loved us, and "washed us from our sins in his own blood",
Romans 3:23 for "all" have sinned and fall short of the glory of God. Rom 6:23 For the wages of sin is death; but the gift of God is eternal life through Jesus Christ our Lord.
Therefore, we need to pay-up or get under Christ's sin payment. Scripture teaches how to do this:
Rom 10:9 That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved. Amen!
David
2 Samuel 6:13 When those who were carrying the ark of the Lord had taken SIX STEPS, he SACRIFICED a BULL and a FATTENED CALF.
(NOTE: Every 6 steps = 2 animal sacrifices. Several search attempts (all different) suggest this trip was between 7-10 miles. Leaving a blood trail of 30k sacrifices? No wonder David dances when the trip is over)