TLDR:
Poirot was always discreetly religious in the books.
The reason why this became more prominent in the series is unknown, but possibly to add more variety to the character to keep him fresh.
Long Answer:
was Hercule Poirot always religious?
Poirot's religion doesn't really feature until the Murder in Mesopotamia, Christie's 14th novel involving him. In it, there are a few sections relating to his religion:
"An interesting man, that Father Lavigny."
"A monk being an archaeologist seems odd to me," I said.
"Ah, yes, you are a Protestant. Me, I am a good Catholic. I know
something of priests and monks."
Also:
Poirot gave a slow appraising glance at us all, then rose to his feet.
I don't know what I expected him to say - something dramatic
certainly. He was that kind of person.
But I certainly didn't expect him to start off with a phrase in
Arabic.
Yet that is what happened. He said the words slowly and solemnly - and
really quite religiously, if you know what I mean.
"Bismillahi ar rahman ar rahim."
And then he gave the translation in English.
"In the name of Allah, the Merciful, the Compassionate."
Finally:
"I had a lengthy conversation with Father Lavigny. I am a practising
Catholic and I know many priests and members of religious communities.
Father Lavigny struck me as not ringing quite true to his role. But he
struck me, on the other hand, as familiar in quite a different
capacity. I had met men of his type quite frequently - but they were
not members of a religious community. Far from it!
Now, the middle quote is obviously the least "obvious", but it shows even the narrator is aware of this religious respect within him.
We hear little else of his religion until the 27th work involving Poirot, The Labours of Hercules, when we are simply told:
Hercule Poirot was a Catholic by birth.
In the 28th work, Taken at the Flood:
"I am a good Catholic," said Poirot cautiously.
Also:
In front of Poirot, set back a little, was the Roman Catholic Church
of the Assumption, a small modest affair, a shrinking violet compared
to the aggressiveness of St. Mary's which stood arrogantly in the
middle of the square facing the Cornmarket, and proclaiming the
dominance of the Protestant religion.
Moved by an impulse Poirot went through the gate and along the path to
the door of the Roman Catholic building.
He removed his hat, genuflected in front of the altar and knelt down
behind one of the chairs. His prayers were interrupted by the sound of
stifled heartbroken sobs.
Finally, from the 41st work, Poirot's Early Cases:
At that time, mon ami, I was, as you know, a member of the Belgian
detective force. The death of M. Paul Dé roulard was not particularly
interesting to me. I am, as you also know, bon catholique, and his
demise seemed to me fortunate.
So, in the book Poirot is certainly religious, although it is fairly discreet. The background to his religion is never really discussed, particularly as so little information is provided regarding his childhood.
do we know why it was glossed over in the original series, and more emphasized in the later episodes
We only have speculation to go on here.
It seems quite probable that it was introduced to stay true to the character from the novels, but also to show a different side of Poirot for followers of just the series. After all, Suchet played the part for a quarter of a century, so I'm sure the addition of this new part of his character was an acting challenge and allowed him to expose a new element of Poirot's character to a modern audience.
Given Suchet's own religious beliefs (he was baptised into the Church of England in 1986) this change was perhaps something he welcomed (or even argued for).
Of course, this is all speculation and in reality we don't know why the change was made.
