“I appeal to you therefore, brothers, by the mercies of God, to present your bodies as a living sacrifice, holy and acceptable to God, which is your spiritual worship. Do not be conformed to this world, but be transformed by the renewal of your mind, that by testing you may discern what is the will of God, what is good and acceptable and perfect.” (Rom. 12:1-2)
The Christian life is not merely assenting to a proposition. It also comes with a life change. God is merciful, he has saved his people by the blood of Jesus Christ; not by the law, not by works, and not by our own strength. When we could not save ourselves and were enemies with God, he saved us. Why? Because of his great mercy, his great love. Our proper response is to give our lives to him.
The irony is that only by giving oneself over completely to God, offering him our very selves, can we truly live. We can attempt to give everything to him, but what we find when we do is that he cannot be out given. When we give our lives, laying them down, taking up our cross daily (Mk. 8:34-36), we find our lives, abundant life in him.
God is glorified in this and we are blessed beyond what we could have imagined. We truly have nothing else to give but ourselves, and this is what the Apostle is urging.
Paul is overcome with the awesome mysteries of God that he has spelled out in the opening eleven chapters, particularly the salvation of the Gentiles. At the end of chapter 11, Paul breaks out in praise to God. He then immediately turns to exhort us in how we should live out this praise.
“By the mercies of God” could qualify anything we do in our Christian lives. We serve him in view of his mercy after being saved in his mercy. We pray to him, worship him, live for him in view of his great mercy, which he demonstrated to us by sending his only Son, Jesus Christ to die for us on the cross and atone for our sin, and through faith in him, we will not die, but have life everlasting (Jn 3:16).
Peter calls Christians a royal and holy priesthood, saved in order to proclaim the excellencies of the God who saved us. Priests offer sacrifices, but we already have the ultimate sacrifice for us in Jesus Christ. What is left but to offer up our bodies, our very lives as a living sacrifice. Why living? Because, unlike the sheep, bull, or pigeon, we have no need to literally die, since we died in Christ. Now it is for us to live in Christ and for God.
This is holy and acceptable to God. He enjoys it and is glorified by it, which is why it is worship. It is true and proper because it can only be done in “spirit and in truth” like the true worshipers God is seeking, as Jesus informed the Samaritan woman at the well.
As Paul reminds us in Galatians 2:20: “I have been crucified with Christ. It is no longer I who live, but Christ who lives in me. And the life I now live in the flesh I live by faith in the Son of God, who loved me and gave himself for me.” This is the reason that our “living sacrifice” is “holy and acceptable.”
And this life is to be transformed into the image of God’s only Son, which is done as the mind is renewed in the living Word of God. It is only here that we can truly know what is “good and acceptable and perfect.”

Before the throne of God above
I have a strong and perfect plea
A great High Priest, whose name is Love
Who ever lives and pleads for me

My name is graven on His hands
My name is written on His heart
I know that while in heaven He stands
No tongue can bid me thence depart
No tongue can bid me thence depart

When Satan tempts me to despair
And tells me of the guilt within
Upward I look, and see Him there
Who made an end of all my sin

Because a sinless Savior died
My sinful soul is counted free
For God, the Just, is satisfied
To look on Him and pardon me
To look on Him and pardon me

Behold Him there, the risen Lamb
My perfect, spotless righteousness
The great unchangeable I Am
The King of glory and of grace

One in Himself, I cannot die
My soul is purchased by His blood
My life is hid with Christ on high
With Christ, my Savior and my God
With Christ, my Savior and my God