"And the LORD spake unto Moses, saying,
If a soul sin, and commit a trespass against the LORD, and lie unto his neighbour in that which was delivered him to keep, or in fellowship, or in a thing taken away by violence, or hath deceived his neighbour;
Or have found that which was lost, and lieth concerning it, and sweareth falsely; in any of all these that a man doeth, sinning therein:
Then it shall be, because he hath sinned, and is guilty, that he shall restore that which he took violently away, or the thing which he hath deceitfully gotten, or that which was delivered him to keep, or the lost thing which he found,
Or all that about which he hath sworn falsely; he shall even restore it in the principal, and shall add the fifth part more thereto, and give it unto him to whom it appertaineth, in the day of his trespass offering."

                            Leviticus 6:1-5 

Restitution is the act of making right sins committed against another individual, and it is something that we are required of God to do after we are saved. These verses in the book of Leviticus should eliminate any doubt in our minds as to whether or not restitution is necessary, for the Lord says that not only should things such as stealing be made right, but also "smaller" thing such as dishonesty ("...or hath deceived his neighbour..."). The seriousness of the sin in our eyes is not important, because in God's eyes all sin is serious.

Sadly though, many Christians today have forgotten the importance of making restitution. It's as if they are willing to confess their sins to God when nobody else is listening, but they don't dare let anybody else know they were wrong.

"...When a man or woman shall commit any sin that men commit, to do a trespass against the LORD, and that person be guilty;
Then they shall confess their sin which they have done: and he shall recompense his trespass with the principal thereof..." 

                            -Numbers 5:6-7

Setting things right with people we have sinned against is important because by not making restitution, we are giving the impression that we don't think we were ever wrong in the first place. How can we honestly expect to be a witness to somebody we have lied to, cheated, and stole from, when we have never made amends with that person?

"Therefore if thou bring thy gift to the altar, and there rememberest that thy brother hath ought against thee;
Leave there thy gift before the altar, and go thy way; first be reconciled to thy brother, and then come and offer thy gift."     

                            -Matthew 5:23-24