ざんげ	33399	46.4%
ザンゲ	12200	17.0%
さんげ	24449	34.0%
サンゲ	1859	2.6%

(this is always ambiguous, maybe さんげ is the assumed default, but then again the kanji are complex)
https://furigana.info/w/懺悔
ざんげ	93.6%
さんげ	3.4%
こひさん	0.9%

https://www.hongwanji.or.jp/mioshie/words/002284.html
Titled： ざんげ. 懺悔
Google translate intro: "Oh, is zange a Buddhist word? Isn't it a Christian word?" I was asked. I see, "zange" does have a Christian feel to it now.

Wikipedia ordering:
https://ja.wikipedia.org/wiki/懺悔
懺悔（ざんげ、さんげ）

https://www.kichijyoin.jp/column/kotoba/detail.php?id=26WL845
　「懺悔」を仏教では「さんげ」と読みます。「ざんげ」というと、主にキリスト教における罪の告白を意味しますので「さんげ」と読むようにしましょう。

Encountered as hiragana ざんげ, as a deathbed confession. Google romanizes the kanji as ざんげ. IME completes on macos for ざんげ, but not at all for さんげ. Same for Android. It would seem the "Christian" interpretation has become predominant, perhaps because of Christianity's emphasis on confession.

"Used elsewhere" vs "other religions" points to the fact this is the "default" reading now, even in non-religious contexts.

@@ -9,0 +10,3 @@
+<reb>ざんげ</reb>
+</r_ele>
+<r_ele>
@@ -13,3 +15,0 @@
-</r_ele>
-<r_ele>
-<reb>ざんげ</reb>
@@ -21 +21,3 @@
-<s_inf>さんげ is usu. used in Buddhism and ざんげ in other religions</s_inf>
+<field>&Christn;</field>
+<field>&Buddh;</field>
+<s_inf>さんげ is usu. used in Buddhism and ざんげ elsewhere</s_inf>

mk

@@ -19,0 +20 @@
+<pos>&vt;</pos>

WP Ja
http://dictionary.goo.ne.jp/leaf/jn/78615/m0u/ざん/

Interesting. Japanese Wikipedia claims that the reading さんげ is used for the Buddhist sense, and ざんげ was later introduced as a reading for the Christian concept.
Daijirin also mentions that the ざんげ reading was introduced sometime during the Edo period, and that さんげ is still used in Buddhism.
The three English words seem suitable for both readings/uses, so could we include a small comment along the lines of "さんげ (usu. Buddhism) / ざんげ (other religions/uses)"?

@@ -20,1 +20,1 @@
-<field>&Buddh;</field>
+<s_inf>さんげ is usu. used in Buddhism and ざんげ in other religions</s_inf>