Mention should be made in SG on this exceptional use, for users to bridge over the gap between typesetting non-latinists, -theologicians and MB cometians.
Rule: Christian trinity is capitalized when directly referred to.
‘Deus’ always capitalized in christian use when referring to the christian god, thus ‘Gloria in excelsis Deo’ but also ‘Credo in unum Deum’. (In very rare cases referring to other gods (e.g. Old Testament), that word for them would be considered thus spelled lowercase.) All reference to Christian trinity and its parts is capitalized, be it direct or by inference: ‘Sanctus’ (Holy [One]), ‘Altissimus’ (Highest [One]) and loanwords with such singular reference (‘[Sanctus Dominus Deus] Sabaoth’, ‘[Christe eleison,] Kyrie [eleison]’); e.g. from Credo: ‘Domine Deus, Rex coelestis, Deus Pater omnipotens. Domine Fili unigenite, Jesu Christe. Domine Deus, Agnus Dei, Filius Patris.’. (N.b. lowercase omnipotens, unigenite, both of which could be argued for by inference.)
Exception is direct address, and I am not sure on this term in English but in ‘I am directly addressing you, God.’, ’you’ be lowercase; e.g. ‘Quoniam tu solus Sanctus, tu solus Dominus, tu solus Altissimus’
‘Pater’ & ‘Agnus’ are capitalized as parts of Trinity, but ‘mater’ (= Maria) is not part, even as ‘Dei mater’ (e.g. ‘Intemerata Dei mater’). Maria however is The special case: ‘virgine’ and ‘mater’ may be capitalized for her, but need not. A special case of this special case is the formal address Beata Maria Virgine (often abbreviated ‘[Missa de] BMV’) and whether this address is qualified by conception, annunciation, 9th hour of crucifixion, ascension, or beatification, I leave for believers.