Why did our Lord Jesus Christ say on the cross “Father, forgive them… ” (Luke 23:34) and did not say by His own authority “your sins are forgiven…”
The Lord Christ on the cross was representing all mankind.
He represented all humanity in paying the wages of sin to the Divine Justice… ” All we like sheep have gone astray; We have turned, every one, to his own way; And the LORD has laid on Him the iniquity of us all.” (Is. 53:6). For this reason, He was on the cross “a burnt sacrifice… a sweet aroma unto the Lord” (Lev. 1:9), and He was a sin offering, and also a “Passover” (1 Cor. 5:7).
He was offering to the Father an atonement for our sins, and as He offered this sacrifice, He said to the Father “forgive them”.
In other words: “I have satisfied the Justice that You, O Father, have demanded, and therefore, forgive them”.
I have paid the wages of sin and shed My blood to redeem them, therefore forgive them”. He spoke as an advocate on behalf of all humanity before the Father, as a representative of every sinner from Adam until the end of all ages.
In His intercession, He was announcing His abdication of His rights toward His crucifiers, those who insulted Him without reason, condemned Him to die unjustly, who falsely accused Him, and stirred the crowd against Him without knowing what they were doing.
He said that as a representative on their behalf as an intercessor for them on the cross.
However, in other circumstances, He performed the forgiveness by Himself as God. He said to the sick man with palsy “Your sins are forgiven” (Mark 2:5) giving the evidence of His Divinity and His authority to forgive sins. Also He said to the sinful woman in the house of Simon the Pharisee “Your sins are forgiven. ” (Luke 7:48). His authority to forgive sins did not depart from Him on the cross, for He forgave the thief on His right, and said to him “Today you will be with Me in Paradise” (Luke 23:43) declaring His forgiveness of his sins, without which he could not enter Paradise.