The Exchange.

One of the soldiers with a spear pierced his side, and immediately there came out blood and water.
Behold the blood of the covenant, which the Lord has made with you. — 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. — It is not possible that the blood of bulls and of goats should take away sins.
Jesus said to them, This is my blood of the new testament, which is shed for many. — By his own blood he entered in once into the holy place, having obtained eternal redemption for us. — Peace through the blood of his cross.
You know that you were not redeemed with corruptible things, as silver and gold, ... but with the precious blood of Christ, as of a lamb without blemish and without spot. Let us draw near with a true heart in full assurance of faith, having our hearts sprinkled from an evil conscience.

One of the strangest things about Christianity, if you are outside looking in, is the idea of the communion service.  Jesus said to His disciples, "Take, eat, this is my body (the bread)"...and, "All of you drink of it (the wine), for this is my blood of the new covenant, which is shed for many." If you look at this as a rational person outside of Christianity, there is no way that you can logically understand why this works.  The reason is that it is the God-appointed way of dealing with our sins.  We cannot receive forgiveness for them, the Scriptures tell us, by doing good works.  No payment is ever enough.  So God devised a plan by which we can receive complete exoneration from all our sins, and be declared innocent--He sent His Son.  Why this works is a divine mystery, but how it works is laid out in the Scriptures from cover to cover.  God allowed His Son to accept the penalty for our sins by dying on our behalf.  

The Son of God replaced us, as it were, at the point of execution, and took the punishment we deserve.  This is good news for those of us who've sinned, and hard to take for those of us who think we are righteous, because we always are attempting to offer up our works in exchange for everlasting life.  God doesn't accept them. He accepts one thing--His own Son's (Jesus') righteousness, brought down into our lives by the simple act of believing that it is true, and that it is true for us.  "He who hears My (Jesus') word, and believes in Him who sent Me, has everlasting life, and shall not come into judgment, but has passed from death into life."

Explain it? Nobody can. Enjoy it and experience it? Anyone is welcome.