Doctrine is simply all the components of Catholic teaching on matters of faith and morals.  It includes teaching that is both learned by study and Divinely revealed.  

Examples of Doctrine:

• Indissolubility of Marriage
• Sanctity of Life
• Matters of Intrinsic Evil
• Male Priesthood
• Liturgical Form 
• Sacramental Matter and Form

A dogma is a component of doctrine which is Divinely revealed, and demands from Catholics their assent of faith.  These are realities about God, His nature, or His actions that are not provable by empirical means and are outside of human reason. By faith one must accept these revealed Truths.  

Examples of Dogma:

• The Immaculate Conception
• The Trinity
• The Two Natures of Christ
• Transubstantiation
• The Assumption

"Whoever listens to you listens to me. Whoever rejects you rejects me. And whoever rejects me rejects the one who sent me.” 

Luke 10:16)

The Church teaches that God is supernatural -- that is, He is beyond comprehension through human means alone.  As mere mortals, we can only grasp completely with our natural intellect the things that are apparent to our natural senses (sight, sound, touch, etc.)  So how did we get to know about God?  Did someone just reason this out?  

Ancient philosophers tried for ages to understand how mankind came into being, and often swept close to the Creator as an abstract idea, but without concrete belief.   

And yet, the yearning for God did not come through an institutional church in the early days of man, but through the instinct infused in him by His Creator.  From the Garden of Eden onward, the story of salvation is the story of God revealing Who He Is to the men and women He has created.  

We call this Divine Revelation.  It's literally God telling us Who He Is and what He's doing and why we need Him above all else.  The Old Testament tells us that God made Himself known to Adam and Eve, and Noah, and Abraham, and Isaac, and Jacob, and so on.  We see this in the first five books of the bible (the Pentatuch or "Torah"), which span over a thousand years of history.  And then another thousand years pass before He becomes man -- the incarnate Jesus Christ -- and saves us from eternal death.  In between, He's revealing Himself to kings and prophets and ordinary people who would become the bearers of His Divine Revelation.  

But Christ not only came to die on the Cross, but to establish a Church -- a Church He built on Peter and his confession that Jesus is "the Son of the Living God".  And that begs the question, "why?"   Why build a Church?  

Because He willed that His revelation would continue to be distilled and taught and interpreted through an organ He explicitly authorized, not only by his own command, but with the infallible assistance of the Paraclete, the Holy Spirit.  And because of this, Jesus has also promised that "the gates of hell will not prevail" against His Church, and that He will "remain with [us] until the end of time."  

Did Divine Revelation Really End?

Formally speaking, yes.  Divine Revelation -- or "public" revelation --  ended with the death of the last Apostle (John).  Christ is the first and final Word of God's utterance -- the Alpha and Omega.  There can be no one who comes after Christ who is more perfect, more informed, more blessed, or has a higher place of glory than the Son of God.  The Church discerned that inspired Scripture ended with John's Apocalypse (Revelation), and anyone who comes forward proclaiming a "new Gospel" or a path of salvation in the person of anyone other than Jesus Christ, is a false prophet.  He is the first word, and the final word.

 
 

If Divine Revelation Ended, Why the New Teachings?

Divine Revelation is multi-faceted in what it reveals and how it reveals it.  The Church is not a static entity, trapped by a surface-only interpretation of everything revealed by God.  The depth of God's teaching is immeasurable, and the Bride is in constant contemplation of Her Groom.  The Holy Spirit guides the Church to deeper and deeper understanding of His Revelation, and so Church teaching blooms larger and larger over time, but without creating anything "new" -- that is, not already present in what God revealed until the death of the Apostle John around 100 A.D.  

For example, there is no explicit Scriptural instruction on the Holy Trinity.  But God has implicitly taught us of the Holy Trinity through what we see in Scripture, and what the Church understood through oral tradition.  The same goes for the Assumption of Mary. You won't find an exact description of such an event in the Bible, but the Church has meditated on and distilled all teaching and all Scripture and, with the help of the Holy Spirit, understood that the Blessed Mother was indeed taken body and soul into heaven.  This is implicitly revealed in several areas of the Old Testament. 

But without a central authority, there would be no way to adjudicate competing ideas of what else is contained in the Deposit of Faith.  The result would be chaos, schism, and heresy, which has left its mark throughout the ages.