The other day I had to fix some rust on the minivan.  It actually surprised me, because I had not noticed a rust problem until I took a closer look.  The metal on the bottom insides of the doors was starting to deteriorate and the culprit- rust.
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Rust is a lot like sin in our own lives.  We might not recognize it at first, but it is still there.  I am reminded of Jeremiah 17:9, “The heart is deceitful above all things, and desperately wicked: who can know it?” 
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Because of the deceitfulness of our hearts we often hide our sins from ourselves like the rust hid on the inside of the van doors.  But like the rust, if sin is not dealt with, it will eventually spread.
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I could have just taken a can of spray paint and painted over the rust.  The rough surface of the deteriorated metal would still be there, but the rust would be hidden- for a while.  That is kind of like what the Old Testament sacrifices did.  They temporarily covered the sin though the consequences of the sin were still there.  They did not really take care of the sin problem though, it just covered it over until Christ could come to take care of it in full.
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Simply painting over the rust would not solve the problem, it would just hide it for a time.  Eventually the paint would bubble and the rust would come back through worse than ever.  I decided to try something different on the van.  I purchased a chemical rust converter that is supposed to actually chemically alter the rust so that it can be painted over and not spread.  I have never used it before so I am not sure it will work, but I do know about something that will work for treating sin. 
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Though the blood of bulls and goats could not take away sins (Hebrews 10:4) Jesus sacrifice did sanctify us (Hebrews 10:10).  But just like the rust, the consequences of the sin are still left behind yet now we are forgiven if we believe on Jesus Christ. 
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If the rust converter really did work on the van, does that mean that new rust will not start in another area?  Of course not.  In a similar way we should not expect that we will never again sin just because we have trusted Christ, but the difference is that now we have something to apply to our sin- His finished payment.  “By the which will we are sanctified through the offering of the body of Jesus Christ once for all.” (Hebrews 10:10)
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This whole concept is clearly explained in the little book of First John.  “If we say that we have no sin, we deceive ourselves, and the truth is not in us.  If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.  If we say that we have not sinned, we make him a liar, and his word is not in us.  My little children, these things write I unto you, that ye sin not. And if any man sin, we have an advocate with the Father, Jesus Christ the righteous:  And he is the propitiation for our sins: and not for ours only, but also for the sins of the whole world.”  (I John 1:8-2:2)
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Sin is a much more serious problem than rust, for the wages of sin is death, but Jesus Christ is far better than rust converter for it is through Him that we get God’s gift of eternal life.  Therefore, believe on Jesus Christ and have your sins converted.