Cain had a nonchalant attitude towards his worship and service of God. He never wanted to do his best for God, he wanted to be allowed to put in whatever is good to him. Hence, his sacrifice and worship were rejected.
As Christians we are in a blood covenant with God. This covenant was established by Christ Jesus and sealed with His blood.
In this Holy covenant is an agreement; God has His own role to play and we as Christians, have our own role to play. God’s ability to keep His part is the faithfulness of God to His Covenant.