There are two ways a body can grow.
A living thing scales by multiplication. A cell divides, and each new cell contains the whole machinery it needs to feed itself, maintain itself, and divide again. Growth adds fully equipped units. The New Testament church scaled this way. A household filled and planted another household, and the new one had the full relational machinery of the old.
An empire scales by annexation. It moves a border. The population inside the new border is added to the whole without being changed by it. It keeps its villages, its customs, its loyalties and its gods, and only the administration is new. Annexation adds subjects to govern without forming a single member.
Scripture ran the experiment first, on Israel, and recorded the result without flinching.
Every ingathering imports gods
Without a single exception, every time the people of God gathered in a new population, that population brought its gods.
It began with Jacob's own household. Then Jacob said unto his household, and to all that were with him, Put away the strange gods that are among you (Genesis 35:2). The founding family was carrying idols in the baggage.
Within weeks of Sinai the exodus generation had made a calf. Stephen puts it exactly when he says they in their hearts turned back again into Egypt (Acts 7:39). They left Egypt, and Egypt's gods travelled along inside them.
Joshua's last assembly came a lifetime later, after the Jordan and Jericho and the whole conquest, and there he said, put away the gods which your fathers served on the other side of the flood, and in Egypt; and serve ye the LORD (Joshua 24:14). At the covenant renewal the command was still put away the gods, because the gods had come the whole way.
The conquest was fenced with warnings about exactly this. They shall not dwell in thy land, lest they make thee sin against me: for if thou serve their gods, it will surely be a snare unto thee (Exodus 23:33). Judges records what happened when the warning was ignored. Its whole first chapter lists the peoples not driven out, and then comes the verdict.
And they forsook the LORD God of their fathers, which brought them out of the land of Egypt, and followed other gods, of the gods of the people that were round about them. (Judges 2:12)
Israel annexed peoples who kept their gods, and the gods flowed uphill into the annexing nation.
The most exact case in Scripture
The most exact instance fits in one verse, where a conquest imports the conquered gods.
Now it came to pass, after that Amaziah was come from the slaughter of the Edomites, that he brought the gods of the children of Seir, and set them up to be his gods, and bowed down himself before them, and burned incense unto them. (2 Chronicles 25:14)
He conquered a nation and adopted the gods of the nation he had just beaten. The prophet's question to him is the question this whole article asks.
Why hast thou sought after the gods of the people, which could not deliver their own people out of thine hand? (2 Chronicles 25:15)
Even in victory, and especially in victory, scale won and the gods came in.
With Solomon the same mechanism ran through alliance instead of war, and the alliances brought their altars. He had seven hundred wives, princesses (1 Kings 11:3), which is to say seven hundred treaties, the ancient method of scaling a kingdom. Every treaty carried a god, until the man who built the temple was building high places for Chemosh and Molech on the hill facing it, and likewise did he for all his strange wives (1 Kings 11:8).
The formula
When Assyria resettled foreign populations into Samaria, they were taught the religion of the land and adopted it. The historian gives his verdict twice so nobody can miss it.
They feared the LORD, and served their own gods. (2 Kings 17:33)
So these nations feared the LORD, and served their graven images. (2 Kings 17:41)
They did both at once. The name of the LORD was added on top and the old gods were kept underneath, and the two ran side by side for generations. That's what a population looks like when it has been added to the people of God without being formed by him.
The church runs the same experiment
Under the household form, entry had a border that formed the people who crossed it. There were years of instruction and sponsors who vouched. At baptism the candidate renounced aloud the devil, his works, and his pomp (his old gods, by name, in public) before he was received. Entry meant putting away the gods at the door, which is Jacob's command made into a rite.
The government form removed the border in three steps. First, entry became advantageous, as the emperor's religion and the path of favor. Then entry became compulsory, with profession required by law and eventually whole peoples baptized by treaty and at swordpoint. Under that flood the border finally collapsed. The years of instruction shrank toward nothing, because you can't catechize an empire, and the words of the renunciation stayed in the rite after the thing they enacted had gone.
So the annexed populations did what annexed populations had done since Jacob's tents. They came in with the baggage.
The sources show Augustine's own mother, newly arrived in Milan, going to the shrines of the martyrs with baskets of bread and wine. These were the old funeral offerings to the dead, carried straight into the church, and she was stopped at the door because the bishop had banned the practice. That convert generation brought the old religion's furniture into the new religion's rooms.
Two centuries later it was policy. Pope Gregory, instructing the mission to the English, directed that the idol temples be left standing and converted, and that the ox-sacrifice feasts continue, redirected to the martyrs' days, because a rude people couldn't be cut off from everything at once. In charity I'd call it pastorally intelligent. Seen with clear eyes, it was a written policy of scaling by retention that kept the places and the feasts and changed the names. It was the Samaria settlement administered from the church's side.
Why distance from God grew as the numbers did
Three reasons compound one another. The first is that formation is relational, and relational capacity has limits. A person is discipled by people who know him, at a table, over years. That doesn't scale by decree. When the government form added populations by the province, it multiplied members without multiplying disciplers. The average member's exposure to formation fell toward zero while his membership was never more secure.
Second, annexation removes the renunciation. Multiplication carries the border with it, since every new household can ask its entrants what they're putting away. Annexation has no door to stand at. When everyone inside the border is a member by law, there's no moment at which the old gods are named and left. Nothing stops them coming in with everyone else.
Third, universal profession destroys the signal. Once everyone is called Christian, the imported gods become invisible, because the word no longer distinguishes the formed from the unformed. In Samaria the historian could still see both religions and name them in one sentence. In Christendom the two wore one uniform, and the old allegiances went where Ezekiel said idols go when they can no longer stand in the open.
Son of man, these men have set up their idols in their heart. (Ezekiel 14:3)
They went inward, into honour, wealth, power and kin-loyalty, where a uniform outward religion hides them perfectly. Eventually the church went from hosting the foreign gods to teaching them.
The test for any era
The principle didn't retire with the emperors. Whenever the church gains people faster than it can form them, whether by law, by fashion, by a movement, by a platform or by a political alliance, ask one question, what did the newcomers renounce at the door?
If the answer is nothing, then the gods came in with them. It didn't take insincere newcomers. The Samaritans feared the LORD, and Monica loved the martyrs. All it took was that nobody stood at a door and asked. A god nobody renounces stays put and acclimatizes.
The design scales like a living thing so that someone can keep the door. Every multiplied household is small enough to know its entrants, ask Jacob's question, and hear Joshua's answer. The smallness that looks like an inefficiency is the customs house. Every form that scaled faster than it could form has smuggled a pantheon, called the cargo Christian, and worshipped it in good conscience for centuries.