God is not holding anyone to a different standard. The standard is always the same: believe what God says. During the OT times, well from Ex 20 onward, it was the Levitical sacrifices. God said do them to be forgiven. They pointed to the ultimate sacrifice of Jesus of Nazareth upon a Roman cross.
Now that the true sacrifice has occured God says to believe in His Son and His sacrifice. It all comes down to believing and trusting in God.
But first, this was only for Israel. We know that gentiles could also repent and be forgiven (the Ninevites, for example).
Second, we can see from the Hebrew scriptures that God can forgive sin even without the Levitical sacrifices. Does God desire sacrifice?
Third, even assuming the divine inspiration of the gospel, does this invalidate God's promises from the Hebrew scriptures?