It is fairly common to hear people speak of God being more severe and harsh under the law of Moses, but more gracious and kind under the law of Christ. This probably comes from some misused applications concerning Nadab and Abihu, Uzzah, Korah, and others who were struck dead by God. However, the writer of Hebrews does not see God as more lenient or more gracious now than under the law of Moses.
Therefore we must pay much closer attention to what we have heard, lest we drift away from it. For since the message declared by angels proved to be reliable, and every transgression or disobedience received a just retribution,how shall we escape if weneglect such a great salvation? (Hebrews 2:1-3; ESV)
The writer of Hebrews presents an argument from the lesser to the greater. I believe the argument is this: If every transgression and every disobedience under the message given by angels (the law of Moses) received just retribution, then how much worse will it be for us, who have been given the message by the Son, if we neglect the great salvation offered? The point is that we will certainly receive a just retribution if we neglect this great salvation. Notice further that the writer does not argue that we will not escape if we also transgress or disobey God’s law. We will not escape if we take the message of the Son lightly (ignore it, neglect it).
The writer of Hebrews does not argue that God is more lenient or gracious to us now. He argues that we have a greater responsibility because we have been given the message of salvation through the Son. With greater responsibility comes greater accountability. If we neglect this great message of salvation, we will not escape a just retribution.