Romans 3:24-26. '....Christ Jesus, whom God set forth as a propitiation by His blood, through faith, to demonstrate His righteousness, because in His forbearance God had passed over the sins that were previously committed, [also] to demonstrate at the present time His righteousness, that He might be just and the justifier of the one who believes in Jesus.'
That's all there is to it really. God could not justly forgive the sins of the O.T. saints, and nor could He forgive the sins of N.T. Christians unless justice was satisfied. The good news is that it has been satisfied by Christ. Praise God!