Redeem means "to buy back." The essential purpose of biblical redemption is to deliver a person or thing from captivity or loss, and as such, it becomes an almost-perfect image for God's saving actions in behalf of sinning mankind. How much would we be willing to pay for the life of someone we love dearly? Kidnappers take perfidious advantage of this desire for the safety of a loved one. They steal a person precious to another—usually a child but sometimes a mate—and hold them for ransom to extort a grand sum of money they think will put them on easy street.
God the Father was willing to pay the ransom price for us by giving up the life of the One He loved most, His own Son, the only other being in all of creation that lived life on the same level as He did. He freely made this sacrifice in exchange for our liberty from our bondage to Satan and our debt to death at the same time. Likewise, the Son willingly volunteered to be the payment in full.
Now, let us turn this reality around and examine it from the perspective of the one released. As one released, how great a sense of loyalty and obligation born out of gratitude do we feel toward the One who came to our rescue by paying such a huge price for our freedom? Plainly and simply stated, this is the issue in regard to our spiritual obligation. This aspect of our salvation is one of the major themes of the book of Ruth. At one point in the narrative, Ruth prostrates herself at her redeemer's feet (Ruth 3:7-14), illustrating her recognition of her obligation.
The book of Philemon relates an interesting event in Paul's life in which he calls upon Philemon's sense of gratitude and obligation to him. In verse 8, Paul says he could use his authority to order Philemon to accept the slave Onesimus back, charging any debt he owed Philemon to Paul. However, he appeals to him through other means. In verse 19, he delivers a double-barreled proposition. First, Paul himself writes in his own hand that he will repay any of Onesimus' indebtedness, putting Philemon in greater-than-normal obligation. Then, Paul reminds him that he owes Paul his very life spiritually. He implies that Philemon's spiritual indebtedness to him should more than cover any material debt Onesimus owed to Philemon.
Therefore, Paul suggests that Philemon charge it to his account. What Paul did for Onesimus reflects in a small way what Christ did for us. As Paul laid himself out for Onesimus, Christ did for us in a much greater way to pay our spiritual indebtedness and set us free. As Paul claims Philemon's indebtedness to him, so Christ claims our indebtedness to Him.