Before the Reformation, this was a vesting prayer said by the celebrant before the Mass began. Archbishop Cranmer moved it to the second prayer of the Communion liturgy (following the Lord’s Prayer) in the Prayer Books.  The celebrant was to pray this kneeling at the Altar Table.  When the Communion liturgy was substantially re-ordered in the 1979 Book, this collect was rendered optional, but was still the second prayer (now following the Acclamation).  The present edition has retained the position of this prayer in the liturgy, returned it to a required piece of the liturgy, but opened it up to be a prayer said also by the congregation rather than only by the minister on their behalf.

Almighty God, to you all hearts are open, all desires known, and from you no secrets are hid:
Cleanse the thoughts of our hearts by the inspiration of your Holy Spirit,
that we may perfectly love you, and worthily magnify your holy Name;
through Christ our Lord. Amen.

When reception of Holy Communion was less frequent, greater efforts were taken by the typical church-goer to prepare for its worthy reception.  Special acts of self-examination and other devotions on the holy mysteries of God’s grace toward sinners were standard fare for Christians of many stripes and traditions.  In this age of weekly Communion as the standard practice, the strictness of preparation and the depths of eucharistic piety have waned.  This prayer, when said by the congregation with the celebrant, reclaims an aspect of historic devotion in preparation for the Sacrament.

The Collect for Purity also provides for the worshiper both instruction and a model concerning right preparation for worship in general.  When we come to worship the Lord, we do not invite God’s presence among us, but rather seek his aid in preparing “the thoughts of our hearts” to enter into his.  God is already with us by virtue of his Word and Spirit; it is we who must be invited and aided to love him perfectly and worthily magnify his holy Name.