Romans 5:11 And not only so, but we also joy in God through our Lord Jesus Christ, by whom we have now received the atonement (Greek = katallagē).
the word "atonement" is translated from the Greek word katallagē which means reconciliation (restoration) as the resulting of Christ exactly (precisely) exchanging His righteousness (blood) for our guilt (HELPS Word-studies).
According to Thayer's Greek Lexicon, katallagē means the restoration of the favor of God to sinners that repent and put their trust in the expiatory death of Christ.
2 Corinthians 5:18-20
18 And all things are of God, who hath reconciled (Greek = katallássō) us to himself by Jesus Christ, and hath given to us the ministry of reconciliation (Greek = katallagē);
19 To wit, that God was in Christ, reconciling (Greek = katallássō) the world unto himself, not imputing their trespasses unto them; and hath committed unto us the word of reconciliation (Greek = katallagē).
20 Now then we are ambassadors for Christ, as though God did beseech you by us: we pray you in Christ's stead, be ye reconciled (Greek = katallássō) to God.
The word "reconciled" in 2 Cor 5:18 and 20 and the word "reconciling" in 2 Cor 5:19 are translated from the Greek word katallássō which means decisively change, as when two parties reconcile when coming ("changing") to the same position.
God (the Party of the first part) established His eternal purpose in eternity past ... and He reconciled Himself to mankind through the death of His Son.
The believer (the party of the second part) is reconciled at the time he/she believes, hence the plea "we pray you, in Christ's stead be ye reconciled " in 2 Cor 5:20. At that point, both parties (God and the believer) came to the same position (both God and the person are reconciled ... i.e. Greek katallássō).
Those who reject God even as He reaches out to them are not reconciled to God ... not because God has not provided all that is necessary for them to be reconciled, but because they reject that which God has provided for them to be reconciled to God.
The Lord Jesus Christ is the reconciliation ... there is no other way.
Faith in the Lord Jesus Christ is reconciliation to God.
If a person rejects the Lord Jesus Christ ... who is the Way, the Truth, the Life ... there is no reconciliation with God because the party of the second part is not in agreement with God (the Party of the first part).
The death of the Lord Jesus Christ was required because without shedding of blood, there is no remission.
Hebrews 9:22 And almost all things are by the law purged with blood; and without shedding of blood is no remission.
... it is the blood of the Lord Jesus Christ that makes atonement.
Leviticus 17:11 For the life of the flesh is in the blood: and I have given it to you upon the altar to make an atonement for your souls: for it is the blood that maketh an atonement for the soul.
.