God has never treated sin lightly because he has always desired to have people reconciled to Himself. Sin can be cured only on the ground of the shed blood of the Son of God. This was true of those who anticipated the death of Christ by animal sacrifices as it is now of those who look back to that death by faith.

Under the old covenant sin was only covered by the blood of animals (Heb 10:4), however it was once and for all removed by Jesus Christ (John 1:29, Col 2:14).

For further study on this you can read "Major Bible Themes" by Lewis Sperry Chafer, chapter 28 (Salvation from the penalty of sin).

James 2:22-24
 23 And the scripture was fulfilled that says, "Abraham believed God, and it was credited to him as righteousness," and he was called God's friend. 24 You see that a person is justified by what he does and not by faith alone.
NIV

Gen 15:6

6 Abram believed the LORD, and he credited it to him as righteousness.
NIV


Abraham looked forward for Christ, we looked back to Christ.