misericordiae ianua

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"O God, Who hast willed that the gate of mercy should stand open to the faithful; look on us, and have mercy upon us; that we who by Thy grace are following the path of Thy will, may never turn aside from the ways of life; through Jesus Christ our Lord."

     William Bright, Ancient collects and other prayers (Oxford:  J.H. & J. Parker, 1857), 73.  Though others have attributed this to the "Leonine" sacramentary (could they have confused it with the Leofric?), Bright attributes it rightly to the 8th-century Gelasian.  It is no. 557 in the Old Gelasian sacramentary (paradigmatically Rome Vatican Reg. 316), no. 951 in the late 8th-century sacramentary of Gellone (Paris, B.N. lat. 12048), no. 749 in the 8th/9th-century Sangallensis (Sankt-Gallen Stiftsbibl. 348), and no. 945 in the 8th/9th-century Gelasian sacramentary of Angoulême (Paris B.N. lat. 816):

"Deus qui misericordiae ianuam fidelibus patere uoluisti, respice in nos et miserere nostri, ut qui uoluntatis tuae uiam donante te sequimur, a uitae numquam semitis deuiemur. Per Dominum nostrum. . . ."

See also p. 104 of the edition of the Old Gelasian as ed. Wilson:

"Deus, qui misericordiae ianuam fidelibus patere voluisti, respice in nos et miserere nostri, ut, qui voluntatis tuae viam donante te, sequimur, a vitae numquam semitis deviemus: per."

"O God, who willed that the gates of mercy should stand open for your faithful, look upon us and have mercy, that as we follow, by your gift, the way you desire for us, so may we never stray from the path of life.  Through."

It is no. 103 in Bruylants (vol. 1, p. 47) and no. 1802 in Corpus orationum.

     Cf. the second of the two alternative Collects for Saturday of the Second Week of Easter, current Roman missal:  

"O God, who willed that through the pascal mysteries
the gates of mercy should stand open for your faithful,
look upon us and have mercy,
that as we follow, by your gift, the way you desire for us,
so may we never stray from the path of life.
Through", etc.

"Deus, qui misericordiae ianuam fidelibus tuis per paschalia mysteria patere voluisti, respice in nos et miserere nostri, ut, qui voluntatis tuae viam, te donante, sequimur, a vitae numquam semitis deviemur. Per Dominum."

Searching the online as well as the scholarly concordances to the "Leonine" or Veronese sacramentary (including both the Initienverzeichnis and the Wortverzeichnis of the critical edition edited by Mohlberg) on "Deus, qui misericordiae" and the word "gate" (ianu* (ianua), ost* (ostium), port* (porta)), I have not found it in the "Leonine", though the phrase "ianuam misericordiae" does occur there in no. 629 (p. 80, l. 17 of the edition of the "Leonine" edited by Mohlberg):

Online:

Scholarly (Bruylants, Concordance verbale des Sacramentaire Léonien, ALMA 18 (1945):  355):

Scholarly (Wilson, Classified index to the Leonine, Gelasian and Gregorian sacramentaries (Cambridge:  1892), 40):

A variation in English:

"O God, who hast willed that the gate of mercy should stand open to the faithful: Look on us, and have mercy upon us, we beseech thee; that we who by thy grace are following the path of thy will may continue in the same all the days of our life; through Jesus Christ our Lord."

    
     It also made its way into the 1906 and 1932 Presbyterian Book of common worship.  Interestingly, the Church of England's Common worship attributes it to Kenneth Stevenson, Bishop of Portsmouth, thus passing over its origin in the Gelasian: 

God of our pilgrimage,
you have willed that the gate of mercy
should stand open for those who trust in you:
look upon us with your favour
that we who follow the path of your will
may never wander from the way of life;
through Jesus Christ our Lord.

     Common worship:  services and prayers for the Church of England (London:  Church House Publishing, 2000), 414 (Post Communion for the Tenth Sunday after Trinity) and 822 (attribution to "The Rt Revd Dr Kenneth Stevenson", but with an asterisk indicating that "the prayer has been adapted" (818)).  Dr. Bridget Nichols, who contributed the chapter in the Companion to Common worship on "Collects and post communion prayers" (ed. Paul Bradshaw, vol. 1 =Alcuin Club collections 78 (London:  SPCK, 2001), 193), agrees that "There is no doubt about the relationship of the prayer you have identified to the Trinity 10 post communion" in Common worship (note in response to me dated 15 March 2015).

     Janus was the god of doors and gates and covered passage- or archways, and therefore of beginnings (commencements; cf. January) and endings (reentries).  Because the Republic especially was so continuously at war, the doors of the Temple of Janus Geminus (Janus the Two-Faced; probably a four-way arch) stood mostly open, but were closed once under Numa, thrice under Augustus, "and more frequently in the imperial period" (OCD⁴, sv Janus, by Nicholas Purcell).

     So does "the gate of mercy" ("misericordiae janua"), standing always open in "the peace of Christ" (misericordia, rather than war or enmity), effect a reversal of the old Roman usage, or is the phrase entirely innocent of any such freight?
     If the allusion is rather to something like Is 60:11,

thy gates shall be open continually: they shall not be shut day nor night.....aperientur portae tuae iugiter die et nocte non claudentur....

then why porta there, but ianua here?  (But of course it must be to something like Is 60:11 as well.)