The Bible often talks about sin as a debt to be paid; and forgiveness as a debt that is discharged (Luke 7:36-50).  Jesus paid for our sin on the cross, experiencing the torments of hell as He was separated from God the Father.  His suffering of body and soul was accepted by Father God as the ultimate and final Passover sacrifice.  As 1 Peter 3:18 says, “For Christ also suffered once for sins, the righteous for the unrighteous, to bring you to God.”  Although the process of dying was started by wicked people, the final act of giving up His spirit was voluntary. Jesus said, “No-one takes it from me, but I lay it down of my own accord. I have authority to lay it down and authority to take it up again." (John 10:18).
 
Jesus was thirsty.  Of course, He was physically dehydrated, but there was another reason for His thirst.  Jesus hated sin and was thirsty to see the righteousness of God exploding through the darkness of a sinful world (Matthew 5:6).  Psalm 63:1 says, “You, God, are my God, earnestly I seek you; I thirst for you, my whole being longs for you, in a dry and parched land where there is no water.”  

Practically, lips moistened by  the wine vinegar enabled Jesus to cry out with a loud voice so that all could hear, “It is finished”.  This was not a desperate last cry of a dying man; it was a common expression used by all trades: it meant ‘Paid in full’.  In love, Jesus had fully paid for all our sin - so that those who repent can be forgiven (John 3:16; Acts 2:38).  
 
The devil's biggest lie is to persuade us that we cannot be forgiven, that we have sinned so badly and God can have no place for us in His heaven.  That is a wicked untruth.  The fact is that Jesus paid for all our sin in full.  No more payment is required.  There is nothing we can contribute to our salvation except to repent and receive it ... and be thankful!  Yet the accusations of Satan are often so strong, convincing us of our unworthiness and of the need to do something good so that we can prove to God that we are worthy.   As the old hymn says, ‘What needless pain we bear’!  All we have to do is to confess our sin and trust Him to cleanse us from unrighteousness (1 John 1:9); and then thank Him as we serve Him.  The reason is that Jesus said our sin is fully paid: and He never lies! (www.crosscheck.org.uk)