Romans 1:2 (the introduction)

which He promised beforehand through His prophets in the Holy Scriptures,

Here is a good example of a Greek mouthful: proepangellmai (it does not even come up on Google) what it means is promised beforehand (weird I know), the reason that Paul felt that he needed to emphasize that for the Gospel of God that he was preaching is that many of the Jews were claiming that this was a new teaching and contrary to what the law and the prophets.  The Old Testament (as we call it) is full of references to what Paul was preaching; even so far back as Genesis 3:15, "And I will put enmity between you and the woman, and between your seed and her seed; He shall bruise you on the head, and you shall bruise him on the heel."  Interestingly enough the word that is translated "bruise" comes from the Hebrew word shuwph (pronounced shoof) and in this context means to crush.  This looks way forward to Jesus' crucifixion.  The devil certainly made quite a strike in killing God the Son but in doing so Jesus crushed Satan's power and paid the debt for our sins.  Also there is Isaiah 1:18; 53; and 55.  In chapter 1:18 it is written, "Though your sins are as scarlet, they will be white as snow; though they are red like crimson, they will be like wool."  There is so much in chapter 53 you might even call it the first Gospel; "Surely our griefs (or sickness) He Himself bore, and our sorrows (or pains) He carried...But He was pierced through for our transgressions, He was crushed for our iniquities; the chastening for well-being fell upon Him, and by His scourging we are healed...but the LORD was pleased to crush Him, putting Him to grief...as a result of the anguish (or toilsome labor) of His soul, He will see it and be satisfied; by His knowledge the Righteous One, My Servant, will justify the many, as He will bear their iniquities." (verses 4a, 5, 10a, 11).  I feel bad about cutting any of it out and strongly recommend that you read the whole chapter.  Also chapter 55:7, "Let the wicked forsake his way and the unrighteous man his thoughts; and let him return to the LORD, and He will have compassion on him, and to our God, for He will abundantly pardon."  That is just a small amount of what the Old Testament has to offer concerning Jesus Christ; there are approximately 325 prophecies concerning Jesus alone (most but not all were fulfilled in the life of Jesus).  His virgin birth (Isaiah 7:14), where He would be born (Micah 5:2), His family's flight to Egypt (Hosea 11:1), the fact that He would live most of His life in Nazareth (Isaiah 9:1,2), the manner of His triumphant entry into Jerusalem (Isaiah 62:11, Zechariah 9:9), even what Jesus cried out from the cross, "Eli, Eli, lama sabachthani that is My God, My God, why have you forsaken me?" (Psalm 22:1); and that is just cherry-picking a few of them.  The list goes on.  The point that Paul was making is that the Gospel that he was neither new nor contrary to the Law, the Prophets, and the Writings (Psalms, Proverbs, Ecclesiastes).

The next part of the verse,"...in the holy Scriptures" reaffirms the authority of scripture.  Interestingly enough when Paul talks about holy Scriptures, he means the Old Testament.  Specifically in his last letter 2nd Timothy 3:14-17, "You, however, continue in the things you have learned and become convinced of, knowing from whom you have learned them, and that from childhood you have known the sacred writing which are able to give you the wisdom that leads to salvation through faith which is in  Christ Jesus.  All Scripture is inspired (or God-breathed) by God and profitable for teaching, for reproof, for correction, for training in righteousness; so that the man of God may be adequate, equipped for every good work."  One of the earliest heretics of the church: Marcion of Sinope (85-160 A.D.) believed in a Dualistic view of Theology (spiritual=good, material=evil) and was one of the earliest believes in what we now call Gnosticism (the belief that salvation comes to special and secret Gnosis or knowledge).  Specifically, Marcion believed that the God of the Old Testament (that he called the demiurge) was the evil creator of matter and was characterized by righteousness and wrath; whereas, the God of the New Testament (specifically Jesus Christ) who is only love and mercy.  He believed that the "teachings of Christ" were inconsistent with the actions of the God of the Old Testament; that does seem to be true if you only skim over both.  In the Old, God pours out wrath, wipes out cities, and allows righteous men like Lot and Joseph to undergo great loss, hardship, and persecution; whereas in the New, God heals, provides, forgives, and shows compassion.  That is, however, until you look deeper.  This is a technical term alert: Immutability.  This attribute of God speaks of His unchanging nature.  The Westminster Shorter Catechism says, ’God is a spirit, whose being, wisdom power, holiness, justice, goodness, and truth are infinite, eternal, and unchangeable.” Those things do not change. A number of Scriptures attest to this idea (e.g. Num. 23:19; 1 Sam. 15:29; Ps. 102:26; Mal. 3:6; 2 Tim. 2:13; Heb. 6:17–18; Jam. 1:17).  So basically God is the same yesterday, today, and forever; God was just as gracious in the Old Testament as He is now and His righteous standard abides forever.  The best example that springs to mind is the account of the Nation of Israel while they were wandering in the desert for 40 years.  You can read about it from Exodus to Deuteronomy; you can barely get a chapter or two before you read of yet another complaint, "What are we going to drink...what are we going to eat...we are sick of this manna, we want meat...why should you be the boss...(and the ever popular) Oh, that we had died in Egypt when [fill in the blank things were better]," and on and on.  Yet God patiently put up with them and graciously provided and protected them.  If you want some good examples of judgement and wrath in the New Testament consider Matthew 23 when Jesus lays into the Pharisees and the Religious Elite; or all the horrible wrath being poured out in the book of Revelation.

Our main problem has always been that we try to judge God based on an external man-made standard; this is both stupid and entirely wrong.  God doesn't do good, He is good; God doesn't love, He is love.  God is in Himself the perfect standard that we should judge by.  In our sinfulness we are still deceived by Satan in the same way he tempted Eve, "Has God indeed said...?).  In the same way as Psalms 14 and 53, "The fool says in his heart 'there is no God.'" we try to reason and justify our thoughts and actions thinking that we (somehow) know better than God.  Fortunately for us, God has given us His Word the Bible so that we can know who He is, where we came from, how to live our lives, and where it is all going.  So too did Paul wright this letter to the Romans to encourage and instruct the faithful.

Not to us, O LORD, not to us,
But to Your name give glory
Because of Your lovingkindness, 
Because of Your truth.
Amen