The Old Testament Sacrificial system where a person brought an animal or other offerings before the Lord was superseded by the Cross. There is no longer a sacrifice needed to atone for sin since the Son of God became sin for us that we could become the righteousness of God. But that doesn't mean we still don't offer sacrifices to the Lord. The biggest sacrifice we can offer to the Lord is obeying Him. This is pleasing to Him and this is where we have fellowship with Him.