Green’s Dictionary of Slang

kneel at the altar v.

(US gay/prison)

1. to fellate.

[US]G. Legman ‘Lang. of Homosexuality’ Appendix VII in Henry Sex Variants.
[US]B. Rodgers Queens’ Vernacular.
[NZ]W. Ings ‘Trolling the Beat to Working the Soob’ in Int’l Jrnl Lexicog. 23:1 64: When a man kneeled to do this [i.e. fellate an anonymous partner] , he was described as doing church or kneeling at the altar.

2. to sodomize.

[US] ‘Jargon of the Und.’ in DN V 453: Kneel at the altar, To commit pederasty.
[US]Irwin Amer. Tramp and Und. Sl. 118: Kneeling at the Altar. – Committing pederasty.
[US]Monteleone Criminal Sl. (rev. edn).
[US]Ragen & Finston World’s Toughest Prison 806: kneeling at the altar – Committing pederasty.
[US]Guild Dict. Homosexual Terms 26: kneel at the altar (v.): The practice of pedication. (Slang.).
[US]R.O. Scott Gay Sl. Dict. 🌐 anal intercourse: [...] Syn: kneeling at the altar.