Do not get me wrong... I believe in the need of the system of faith. Since faith and believing are pretty much interchangeable terms - faith is believing and believing is faith - they are an important component in Salvation. However, my point here is that Justification of the Elect is not dependent upon our faith. Romans 3:21-31, is the revelation of God's faith in His purpose and His Son's completed work.
Does God have faith? Of course He does. He established the concept and ability in His Created beings. In Roman's 3:25:
whom God set forth to be a propitiation, through faith, in his blood, to show his righteousness because of the passing over of the sins done aforetime, in the forbearance of God;
The term "through faith", goes back to the Noun - Theos - God. The verse is saying, God through faith, in the shed blood of Jesus Christ - could pass over the sins of the OT Saints before the actual event took place in history. This comes as no shock to the ones who believe in the Doctrines of Election and Predestination. We are used to the concept of things being completed from Eternity, even before the foundation of the world.
Paul was beginning the argument of "faith" from it's source - God. He goes on to the "system of faith" in Rom.3:28
Rom 3:28 We reckon therefore that a man is justified by faith apart from the works of the law.
This Justification is not by anything the believer does or does not do, but is through the "system of faith" established by God and comes directly from God's faith. This is the beautiful answer, to how God could impute righteousness to the OT Saints even before Christ's first advent. God being Sovereign over His creation and being the one and only god, what would stop His purpose or His faith in it.
Paul, having established the understanding of God's Justification of the Elect and this system of faith, then moves to it's application, in Romans 4. That through this system of faith, Abraham was considered Righteous.
Rom 4:3 For what saith the scripture? And Abraham believed God, and it was reckoned unto him for righteousness.
Abraham was already a believer, was he not? We know what must take place before a fallen man can believe God. Being foreknown, being predestinated, being called and being Justified. All past tense verbs - Abraham, like all of the Elect, was already Justified, taking us back to Romans 3. Abraham experienced the "New Birth" at the appointed time and heard the Lord and believed the Lord and His Truth. Therefore, believing the Lord was an act of righteousness. But his act of righteousness could no more save him than it could save us.