同質說（Consubstantiation），亦稱為合質說或聖體共在論，[1]是基督教神學中有關聖體實在的理論之一。與聖餐變體論（Transubstantiation）相反，它相信聖餐禮中基督的肉與血是和麵包與酒同在的，而不是通過彌撒轉換而成的。此學說是羅拉德派的核心思想之一，天主教視其為異端。一些信義宗信徒與牛津運動者認同這種觀點，但教會、認信文和護教學家都不贊成[2][3][4]；信義宗將其聖餐觀稱作「真實臨在」說，差異為前者可能包含「基督的血與葡萄酒物質上混合在一起」的教導，而後者認為「主基督真正的身體和血，在餅和酒之中，亦在餅和酒之下」，不與物質實體相混但又真實臨在[5]。
在早期教會中也存在類似觀點。150年左右游斯丁就曾說過：「這些被神祝福過的、滋養我們的血與肉的食物（麵包和葡萄酒）也是肉身化的耶穌的血與肉」。[6]
《宗教改革對禮儀的
影響與反思》，天國事齊探討：宗教改革五百年，頁14
Formula of Concord, Epitome, VII.42 and Solid Declaration VII.127 in F. Bente, Triglot Concordia, (St Louis: CPH, 1921), 817, 1015.
Real Presence Communion – Consubstantiation?. WELS Topical Q&A. Wisconsin Evangelical Lutheran Synod. [26 Jan 2015]. （原始內容存檔於2008-01-02）. Although some Lutherans have used the term 'consbstantiation' [sic] and it might possibly be understood correctly (e.g., the bread & wine, body & blood coexist with each other in the Lord's Supper), most Lutherans reject the term because of the false connotation it contains...either that the body and blood, bread and wine come together to form one substance in the Lord’s Supper or that the body and blood are present in a natural manner like the bread and the wine. Lutherans believe that the bread and the wine are present in a natural manner in the Lord’s Supper and Christ’s true body and blood are present in an illocal, supernatural manner.
Lectures on the Augsburg Confession. Theological Seminary of the United Lutheran Church in America (Lutheran Publication Society). 1888: 350 [13 June 2014]. （原始內容存檔於2021-03-11）. But in neither sense can that monstrous doctrine of Consubstantiation be attributed to our church, since Lutherans do not believe either in that local conjunction of two bodies, nor in any commingling of bread and of Christ's body, of wine and of his blood.
