As the Westminster Confession also says: 'The moral law does forever bind all, as well justified persons as others, to the obedience thereof, and that, not only in regard of the matter contained in it, but also in respect of the authority of God the Creator, who gave it. Neither does Christ, in the Gospel, any way dissolve, but much strengthen this obligation. (Ch XIX, Para V)
God's law reveals God's nature, and His sacrificial law having prophesied Christ and been fulfilled in His perfect blood, His righteous moral, civil and physical laws which govern the universe cannot pass while the earth remains (Gen 8:22; 2Pet 3:7) And even in the new heavens and earth, God's righteousness will still be there (2Pet 3:13).
Plainly, we believers, repentant sinners as we are, needed Christ to keep the law where we could not that He might justify us. 'And ye know that he was manifested to take away our sins; and in him is no sin' (1John 3:5). But the Lord cannot then lead our lives for us. All have to 'work out your own salvation, with fear and trembling' (Phil 2:12), and 'run with patience the race that is set before us' (Heb 12:1) keeping the moral law as Christ commanded.
The Bible says we shall all stand before the judgment seat of Christ and give an account of ourselves (Romans 14:10,12). In that day, it will not be enough to say, 'But Lord, I thought you fulfilled the requirements of the moral law for me.' No, we have to keep it ourselves.
When the lawyer asked Jesus how to get eternal life, the Lord asked him what was written in the law (Luke 10:25ff). The lawyer correctly quoted Deut 6:5 and Lev 19:8. The Lord said, 'This do and thou shalt live.' He did not say, 'I'll fulfil that for you, so don't bother keeping it because you can't.' Actually, by the indwelling of the Holy Spirit and by the grace of God and the risen power of Christ, we can! 'Whoever is born of God doth not commit sin.' (1John 3:9)
'For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins, but a fearful looking for of judgment and fiery indignation, (Heb 10:26-27).