All of the Old Testament patriarchs and prophets looked forward to the time of Christ. King David perhaps above all of them. Why? Because David had a revelation of grace that was unparallel in the Old Testament. He spoke in the Psalms of the time which we live now. He looked forward to it, saw what we have and called us supremely blessed.
Why did David speak of the blessedness of the New Testament man? Because he saw that the time was coming when believers would live without sin charged against their accounts. He prophesized that all iniquity would be forgiven, sins would be covered and completely forgotten.
In fact, in this passage, there are two important truth to be aware of, to which Paul draws our attention. The word for 'will not', or 'will never' in verse eight is what is called an emphatic negative in the Greek. It means that sin, will never, not ever, be counted against us. It is the strongest language that could be used to describe the state of the New Testament believer. Our sins have been forgiven to the point that they will never be held against us ever again. This is incredible news for us and David calls this man, those who live after the promised Messiah, blessed, happy and to be envied by all.
The second word is also found in verse eight which says, "Blessed is the man whose sin the Lord will never count against him." or as the King James Bible says, "Blessed is the man to whom the Lord will not impute sin." The word impute is actually an accounting term, meaning to account for, charge against or reckon to. An example of this is when we use our credit card. The amount charged is imputed to our account, of which, we later have to consider by paying off the debt. However David speaks of the man to which the Lord will never charge into their account, the iniquity that has been committed. Even when a sin is committed presently the Lord does not impute it to us. This speaks of past, present and future sin because Jesus has paid the price for sin once for all. ( Hebrew 10:10; Hebrews 10:14 ) Again great news for all believers that live on this side of the cross.
Paul used two examples of great men of faith from the Old Testament scriptures to support his claim to the Jews that we are completely justified by faith apart from works. Both Abraham and David knew this truth and looked forward to it's fulfillment. David, in particular was speaking of the New Testament believer, which is us. This blessedness that he envied is what is we currently possess in Christ. Today praise God that this includes you! You have been bought with a price, you have been redeemed completely and fully forgiven of every offense ever committed. This is good news both for the Jew and for the Gentile through the blood of Christ Jesus!