Not so difficult really.
And there are three possible explanations.
1. There was another John Richards with wife Juliana
2. The 1777 baptism did not take place until the child was about 1 year old
3. The 1777 baptism was a private baptism and 1778 the public baptism
Third option is the most likely but unfortunately it is not reflected in the copy of the register available from FamilySearch.
The handwriting indicates that this register is a transcript of the original so it is quite possible the transcription was not as faithful and accurate as it might have been.
The burials you need in case you don't have them:-
Charles son of John Richards buried 17th June 1775
Charles Richards buried 14th March 1780
During the period 1776 to at least the late 1780s, except on extremely few occasions, only the names of those buried were recorded. No indication of the deceased being a child or, in the case of a female, the 'wife of' or 'widow' as can be found in many other Parishes.
Identifying any John Richards is always a problem but that does not mean it cannot be done.
Knowing when John died would be of help provided his age was recorded when he was buried but in many Parishes that did not happen much before 1813.
But we do have some information that might help.
1. Juliana Batten appears to have been baptised in 1739 at Madron to Ralph Batten
2. Children of John and Juliana - John, Jane, Elizabeth, Charles, Charles, Charles
3. One of the witnesses to the marriage was Alexander Richards
I would suggest that your John Richards was most likely the son of John Richards and Margaret Battin who married at Paul in 1726.
There was another John Richards at Paul at the same time and he married Frances Cruzelier also in 1726.
It is difficult to correctly determine all of the children for each couple because prior to 1741 only the father's name was recorded in the baptismal register and there were other John Richards married in 1712 and 1715 who could have been baptising children at least to the 1730s.
However, working on the few clues we have I think I can mount a reasonable case for you.
John Richards of Paul married Margaret Battin at Madron 5th November 1726
Alexander son of John Richards baptised 17th March 1726 Paul (old style date)
* father's name only for the next few years but those children not relevant at the moment *
Charles son of John and Margaret Richards baptised 18th May 1741 Paul
Joseph son of John and Margaret Richards baptised 7th March 1743 Paul (privately)
Martha daughter of John and Margaret Richards baptised 14th November 1745 Paul
Ralph son of John and Margaret Richards baptised 4th February 1746 Paul (old style date)
There are two John Richards baptised at Paul to John during that period from 1726-1740 and one will belong to this family.
John son of John Richards baptised 28th June 1729 Paul
John son of John Richards baptised 27th January 1733 Paul (old style date)
It is also possible that there may have been some children baptised at Madron during that period from 1727-1740 but again only the father's name would have been recorded. In fact there was a Charles son of John Richards baptised at Madron in 1729 but no indication that he might have been from Paul.
Nevertheless - in the scenario above we have Alexander Richards as the likely eldest son of John and Margaret Richards and then in 1766 he witnesses the marriage of his brother.
We also have the name Charles in the family so you need to wonder just what it was about him that prompted brother John to ensure he had a surviving son of that name.
And I think you might find John did this because his brother Charles also died in 1775 - a Charles Richards was buried 11th December 1775 just six months after John's son.
It would be interesting to find out what caused the death of Charles but I think there is enough circumstantial evidence her now to suggest that your John was the son of John Richards and Margaret Battin.
Now to the Battin/Batten family for a moment. If my theory is correct then we have a scenario that suggests Margaret and Juliana Batten might also be related.
I have already mentioned that Juliana was the daughter of Ralph Batten and then the last child of John and Margaret was named Ralph.
Based on Transcribed records and the most likely baptisms for Margaret (wife of John) and Ralph (father of Juliana) this is what I think the Batten family looks like:-
John Batten married
?? sometime before 1700
Martha daughter of John Batten bp. 18th August 1700 Paul
Margaret daughter of John Batten bp. 25th July 1703 Madron
Ralph son of John Batten bp. 2nd February 1705 Madron (old style date)
Mary daughter of John Batten bp. 16th April 1709 Madron
Elizabeth daughter of John Batten bp. 1st November 1713 Madron
There were only few Battens at Madron during this period with only one Margaret and two Ralphs baptised. The other Ralph was the son of William Batten but he died soon after birth.
So John Richards married Margaret Batten and their son John then married Margaret's niece Juliana, daughter of brother Ralph.
That might all seem a little confusing but work your way through it and if you have any queries don't hesitate to ask.
CT