Learn French: Sa conviction est forte, cela explique le jugement. - His conviction is strong, that explains the judgment.

In French, cela is used to refer back to a concept, idea, or statement that was just mentioned. Because it points backward to the first half of the sentence (Sa conviction est forte), cela is the natural choice. Ceci is usually used to point forward to something you are about to say or introduce.

Would it be wrong to use ça instead of cela here?

It would not be grammatically wrong to say ça explique le jugement, but ça is mostly used in spoken and informal French. Cela is preferred in written language or more formal registers, which suits the slightly formal vocabulary of this sentence (conviction, jugement).

If the translation says 'His conviction', why does the French use the feminine sa?

Remember that in French, possessive adjectives agree with the thing being owned, not the owner. Because the word conviction is feminine, you must use sa, even if the person who holds the conviction is a man. Out of context, sa conviction could mean 'his conviction' or 'her conviction'.

Why can't we use il to mean 'it' or 'that' in the second clause?