People didn't keep the 613 commandments before Sinai and they didn't keep them after Christ. The dispensation of Law has ended, therefore, we don't have to obey the Law now.
When presenting this objection, no Scripture has been presented to support it. Interestingly enough, Scripture states the exact opposite.
In speaking to Isaac, God declares the following:
"I will multiply your descendants as the stars of heaven, and will give your descendants all these lands; and by your descendants all the nations of the earth shall be blessed; because Abraham obeyed Me and kept My charge, My commandments, My statutes and My laws."
Apparently, people did keep God's Law before Sinai! Abraham, the man of faith according to Hebrews 11, kept God's commandments, statutes, and laws.
And it appears that we will be keeping the commandments when Messiah returns:
Now it will come about that in the last days The mountain of the house of the LORD Will be established as the chief of the mountains, And will be raised above the hills; And all the nations will stream to it. And many peoples will come and say, "Come, let us go up to the mountain of the LORD, To the house of the God of Jacob; That He may teach us concerning His ways And that we may walk in His paths." For the law will go forth from Zion And the word of the LORD from Jerusalem. (Isaiah 2:2-3)