For if that first covenant had been faultless, then no place would have been sought for a second. (Hebrews 8:7)
The first covenant (Old Testament) was and is not flawed, man's willingness and ability to keep it was and is flawed. Therefore God in His mercy and graciousness has made a new covenant (New Testament) by which He is not only the covenant maker, but the covenant keeper as well. The first covenant priesthood was unable to atone for sin because of their sin. The priest of the new covenant could because He was and is sinless. And naturally, that high priest is Christ. The first covenant is not null and void. The laws of God are unchanging. It is just that salvation is no longer conditional to it. Salvation is now, by the new covenant, based on faith in the only one able to keep the covenant, Christ Jesus. The first covenant was based on works of the law the second is of faith in the lawgiver. Therefore, even those who were under the old covenant received salvation by placing their faith in the coming of Messiah, the author of the new covenant.