The Trinity as a word entered Middle English in the 13th century, and was associated with Holy and similar words from the start. Holy Trinity would first appear uninterrupted around 1300.

The Middle English Dictionary entry cites an early instance from about 1250 as written in a manuscript of the Ancrene Riwle, a guidebook for anchoresses (or female anchorites):

Tu ʒette me ham, holi þrumnesse, trinite [Pep: Trinete], iþe wurðschipe of þe.

 

You get me home, holy Three-ness, Trinity, in the worship of you. (rough translation - unsure about first tu and distrusting of iþe)

The use in the Ancrene Riwle is remarkable because some of the first attempts to write clerical rules in Middle English were for women pursuing spiritual devotion, including anchoresses. The Ancrene Riwle, the texts of the Katherine Group, and other writings were explicitly addressed to women audiences. Despite the interruption of an earlier word for Trinity (thrumness or three-ness), the close coincidence of holi and trinite show the words are already associated.

ETA and here the phrase appears in the closely-related Ancrene Wisse, dated by the editor Robert Hasenfratz to be from 1225-1240:

Ther the Hali Trinite - "thrumnesse" on Englisch - schawde hire al to him: the Feader in his stevene, the Hali Gast i culvre heow, the Sune in his honden. (Part 3, lines 464-466)

 

There the Holy Trinity - "threeness" in English - showed herself completely to him: the Father in his voice, the Holy Spirit in the dove's shape, the Son in his hands. (editor's translation)

Around 1275, the usage appears again in the medieval (and fanciful) chronicle of British history, The Brut:

Anan he gon to wurche ane swiðe feire chirche a seinte Trinetðes nome. (lines 14739-40)

 

Anon, he goes to build an exceedingly fair church in the holy Trinity's name.

This part of the narrative describes the actions of King AEthelbert of Kent , converted by St. Augustine of Canterbury at the start of the 7th century. Seinte functions as an adjective that means something like holy or sacred.

Finally, by 1300 the appelation appears in a form close to our own Holy Trinity in the South English Legendary, a collection of saint's lives:

Lo here..gret bi-tokningue of þe holie trinite, Of fader and sone and holie gost bi þeose ʒeorde þreo. (233-4)

 

See here ... a great sign of the Holy Trinity, of Father and Son and Holy Spirit by these three a garden. (Rough translation)

From this time, the word appears much more widely and consistently, in ways documented by Sven Yargs's answer.

The Trinity as a word entered Middle English in the 13th century, and was associated with Holy and similar words from the start. Holy Trinity would first appear uninterrupted around 1300.

The Middle English Dictionary entry cites an early instance from about 1250 as written in a manuscript of the Ancrene Riwle, a guidebook for anchoresses (or female anchorites):

Tu ʒette me ham, holi þrumnesse, trinite [Pep: Trinete], iþe wurðschipe of þe.

You get me home, holy Three-ness, Trinity, in the worship of you. (rough translation - unsure about first tu and distrusting of iþe)

The use in the Ancrene Riwle is remarkable because some of the first attempts to write clerical rules in Middle English were for women pursuing spiritual devotion, including anchoresses. The Ancrene Riwle, the texts of the Katherine Group, and other writings were explicitly addressed to women audiences. Despite the interruption of an earlier word for Trinity (thrumness or three-ness), the close coincidence of holi and trinite show the words are already associated.

ETA and here the phrase appears in the closely-related Ancrene Wisse, dated by the editor Robert Hasenfratz to be from 1225-1240:

Ther the Hali Trinite - "thrumnesse" on Englisch - schawde hire al to him: the Feader in his stevene, the Hali Gast i culvre heow, the Sune in his honden. (Part 3, lines 464-466)

There the Holy Trinity - "threeness" in English - showed herself completely to him: the Father in his voice, the Holy Spirit in the dove's shape, the Son in his hands. (editor's translation)

Around 1275, the usage appears again in the medieval (and fanciful) chronicle of British history, The Brut:

Anan he gon to wurche ane swiðe feire chirche a seinte Trinetðes nome. (lines 14739-40)

Anon, he goes to build an exceedingly fair church in the holy Trinity's name.

This part of the narrative describes the actions of King AEthelbert of Kent , converted by St. Augustine of Canterbury at the start of the 7th century. Seinte functions as an adjective that means something like holy or sacred.

Finally, by 1300 the appelation appears in a form close to our own Holy Trinity in the South English Legendary, a collection of saint's lives:

Lo here..gret bi-tokningue of þe holie trinite, Of fader and sone and holie gost bi þeose ʒeorde þreo. (233-4)

See here ... a great sign of the Holy Trinity, of Father and Son and Holy Spirit by these three a garden. (Rough translation)

From this time, the word appears much more widely and consistently, in ways documented by Sven Yargs's answer.

The Trinity as a word entered Middle English in the 13th century, and was associated with Holy and similar words from the start. Holy Trinity would first appear uninterrupted around 1300.

The Middle English Dictionary entry cites an early instance from about 1250 as written in the Ancrene Riwle, a guidebook for anchoresses (or female anchorites):

Tu ʒette me ham, holi þrumnesse, trinite [Pep: Trinete], iþe wurðschipe of þe.

You get me home, holy Three-ness, Trinity, in the worship of you. (rough translation)

The use in the Ancrene Riwle is remarkable because some of the first attempts to write clerical rules in Middle English were for women pursuing spiritual devotion, including anchoresses. The Ancrene Riwle, the texts of the Katherine Group, and other writings were explicitly addressed to women audiences. Despite the interruption of an earlier word for Trinity (thrumness or three-ness), the close coincidence of holi and trinite show the words are already associated.

Around 1275, the usage appears again in the medieval (and fanciful) chronicle of British history, The Brut:

Anan he gon to wurche ane swiðe feire chirche a seinte Trinetðes nome. (lines 14739-40)

Anon, he goes to build an exceedingly fair church in the holy Trinity's name.

This part of the narrative describes the actions of King AEthelbert of Kent , converted by St. Augustine of Canterbury at the start of the 7th century. Seinte functions as an adjective that means something like holy or sacred.

Finally, by 1300 the appelation appears in a form close to our own Holy Trinity in the South English Legendary, a collection of saint's lives:

Lo here..gret bi-tokningue of þe holie trinite, Of fader and sone and holie gost bi þeose ʒeorde þreo. (233-4)

See here ... a great sign of the Holy Trinity, of Father and Son and Holy Spirit by these three a garden. (Rough translation)

From this time, the word appears much more widely and consistently, in ways documented by Sven Yargs's answer.

The Trinity as a word entered Middle English in the 13th century, and was associated with Holy and similar words from the start. Holy Trinity would first appear uninterrupted around 1300.

The Middle English Dictionary entry cites an early instance from about 1250 as written in a manuscript of the Ancrene Riwle, a guidebook for anchoresses (or female anchorites):

Tu ʒette me ham, holi þrumnesse, trinite [Pep: Trinete], iþe wurðschipe of þe.

You get me home, holy Three-ness, Trinity, in the worship of you. (rough translation - unsure about first tu and distrusting of iþe)

The use in the Ancrene Riwle is remarkable because some of the first attempts to write clerical rules in Middle English were for women pursuing spiritual devotion, including anchoresses. The Ancrene Riwle, the texts of the Katherine Group, and other writings were explicitly addressed to women audiences. Despite the interruption of an earlier word for Trinity (thrumness or three-ness), the close coincidence of holi and trinite show the words are already associated.

ETA and here the phrase appears in the closely-related Ancrene Wisse, dated by the editor Robert Hasenfratz to be from 1225-1240:

Ther the Hali Trinite - "thrumnesse" on Englisch - schawde hire al to him: the Feader in his stevene, the Hali Gast i culvre heow, the Sune in his honden. (Part 3, lines 464-466)

There the Holy Trinity - "threeness" in English - showed herself completely to him: the Father in his voice, the Holy Spirit in the dove's shape, the Son in his hands. (editor's translation)

Around 1275, the usage appears again in the medieval (and fanciful) chronicle of British history, The Brut:

Anan he gon to wurche ane swiðe feire chirche a seinte Trinetðes nome. (lines 14739-40)

Anon, he goes to build an exceedingly fair church in the holy Trinity's name.

This part of the narrative describes the actions of King AEthelbert of Kent , converted by St. Augustine of Canterbury at the start of the 7th century. Seinte functions as an adjective that means something like holy or sacred.

Finally, by 1300 the appelation appears in a form close to our own Holy Trinity in the South English Legendary, a collection of saint's lives:

Lo here..gret bi-tokningue of þe holie trinite, Of fader and sone and holie gost bi þeose ʒeorde þreo. (233-4)

See here ... a great sign of the Holy Trinity, of Father and Son and Holy Spirit by these three a garden. (Rough translation)

From this time, the word appears much more widely and consistently, in ways documented by Sven Yargs's answer.