Ontario, Roman Catholic Church Records, 1760-1923 are online at FamilySearch which is wonderful.
The caveat is that they are not indexed. Researchers can find it quite a task to go through image after image looking for your ancestor.
I was searching in Lennox and Addington, Centreville, St Anthony of Padua and discovered that the Priests actually kept an index to the baptisms. The image on the left shows the start of the indexes. There are 230 images and image 3 is the start of the index.
To use the image you find the name of the individual you are seeking. on the left of the name is a number which is the number of the baptism in the register. To the right of the name is another number which is the page number.
The register is described on FamilySearch as Baptisms 1844-1889 but the first baptisms begin 24 February 1856.
The first baptism is found on Image 32 and is dated February 24, 1856 for little Jeremiah Gaffney who was born June 25, 1855 in Hungerford.
However if you go to the back of the register book there are earlier baptisms. Image 186 starts baptisms in 1844. The heading on the page states "The following copied from the Parish of Railton Records"
The detail in this church register is amazing. For example for little Jerermiah his date of marriage, place of marriage and the name of his bride as well as her parents are noted in the Remarks column. His date of death are also noted.