By development of doctrine, we mean that some divinely revealed truth has become more deeply understood and more clearly perceived than it had been before. Under the guidance of the Holy Spirit, whom Christ promised to send to teach us, the Church comes to see more deeply what she had always believed, and the resulting insights find expression in devotion of the faithful that may have been quite uncommon in the Church's previous history. The whole spectrum of Christology and Mariology has witnessed such dogmatic progress...
Always implied in such progress is that, objectively, the revealed truth remains constant and unchanged. But through the light of the Holy Spirit, the subjective understanding of the truth becomes more clear, its meaning becomes more certain and its grasp by the believing mind becomes increasingly more firm. Only the subjective grasp of men increases, without the actual doctrine or dogma changing in an essential way. This is the main distinction to keep in mind when considering development.
Like many Christian doctrines, the idea of doctrinal development is based on much implicit or indirect scriptural evidence. The best indications are perhaps Mt 5:17, 13:31-32, Jn 14:26, 16:13, 1 Cor 2:9-16, Gal 4:4, Eph 1:10, 4:12-15. Furthermore, doctrine clearly develops within Scripture itself (“progressive revelation”).
Some examples would be: doctrines of the afterlife, the Trinity, the Messiah (eventually revealed as God the Son), the Holy Spirit (a Divine Person in the New Testament), the equality of Jews and Gentiles, bodily resurrection, sacrifice of lambs evolving into the sacrifice of Christ, and so forth. Not a single doctrine emerges in the Bible complete with no further need of development.
In general, whenever Holy Scripture refers to the increasing knowledge and maturity of Christians and the Church, an idea very similar to doctrinal development is present. Holy Scripture, then, is in no way hostile to development.
The canon of Scripture itself is an example of developing doctrine. The New Testament never informs us which books comprise itself, and its canon (final list of books) took about 360 years to reach its final form (at the Council of Carthage in 397). For instance, the books of Hebrews, James, 2 Peter, 2 and 3 John, Jude, and Revelation were not widely accepted by the Church until 350 A.D.
The Church is called the “Body” of Christ often (e.g., Eph 1:22-3), and is compared to a seed that grows into a tree (Mt 13:31-2). Seeds and bodies grow and expand. Yet Protestants tend to see Church and doctrine as more like a statue, subject to pigeon droppings (i.e., so-called Catholic “corruptions”!). This robs the metaphors of Christ of their essential meaning.
It is impossible to claim that no development occurred in Church history, or that it ceased after the 1st, 2nd, 3rd, or 5th century, etc. (all arbitrary human traditions). The Bible is not absolutely clear in every part, and requires the developing wisdom of the Church.
Doctrines agreed upon by all develop, too.
The Divinity or Godhood of Christ was only finalized in 325 at the Council of Nicaea,
and the full doctrine of the Trinity in 381 at the Council of Constantinople.
The dogma of the Two Natures of Christ (God and Man) was proclaimed in 451 at the Council of Chalcedon. These decisions of General Councils of the Church were in response to challenging heresies.
Although understanding increases, the essential elements of doctrines exist from the beginning. Today’s Church shouldn’t be expected to look like the primitive Church if it is a living, vibrant, spiritual organism.
Indeed, there is a lot of Scripture that would indicate the Trinity and Christology (Jesus as God), but if one doesn’t attempt to put the verses together in a certain systematic way, it wouldn’t jump right out from Scripture. So for that reason the Church had to develop it — and usually in response to heretics.
So, for example, the heretical Nestorians came around and claimed that Jesus was two persons: human and divine. The Church said “no, that’s not true. He is one Person, the God-Man.” The Nestorians speculated falsely about the nature of Mary, but the Church pronounced on the doctrine of Mary, against the Nestorians, giving her the title of Theotokos, which means “God-bearer,” or “Mother of God.” That occurred at the Council of Ephesus in 431. We often find, then, in Church history, heretics coming along and making a new claim. The Church reflects upon it, and rules against it.
Christology was further elaborated upon twenty years later, at the council of Chalcedon in 451. This Council promulgated the notion of the Two Natures of Christ and the Hypostatic Union:, that is, Jesus is God and Man, with no separation. That was in response to the Monophysite heresy, which held that Jesus had one nature. The Church ruled that He had Two Natures. And so, on and on, with all the heretics. The Bible itself, in its own example, demonstrates development.