Before the coming of the messiah; Jesus, God set up guidelines for the Jews (alone) to use as a way of getting to know Him and getting a temporary reprieve from sins. The laws were for jews and not forced upon anybody, even the jews themselves as they chose at Mt. Sinai to do all there was therein.
When Jesus came, He bore our sins and became the sacrificial lamb through whom by faith in Him we gain salvation from sins Once and for All. So they was no need for as to go through all the specific aspects of the law anymore to gain salvation and a Way to the Father.
Jesus Christ is God's Son not by human will but by incarnation of God into his being; God's begotten. That makes faith in what He did on the cross so powerful because His blood is not like that of goats and bulls sacrificed in the jewish temple but a product of God's incarnation into the Person of Christ!
Therefore by faith in Him and not our futile selves and efforts or by sacrifices and rituals of the old Testament or any modern type that has sprung up today in various religions, we gain freedom from sin!
The old Testament is by no means done away with but its practices for salvation and freedom from sin are no longer needed. It however still gives us an insight into the mind of God and the beauty of the cross and Jesus!

New Testament makes Old Testament simple through Jesus Christ.