After the Collect for Purity comes what may be termed the Penitential Rite, consisting of either the Decalogue (BCP 100) or the Summary of the Law.  The Summary of the Law entered the Prayer Book tradition in 1790 as an option to follow the Decalogue, basically offering a New Testament summary of the Old Testament Law.  The Kyrie followed the Summary of the Law.  Later editions of the American Prayer Book allowed for the Summary of the Law and Kyrie to be said without the Decalogue, provided that the Decalogue was read at least once each month.  The following collect also came to follow the Penitential Rite:

O ALMIGHTY Lord, and everlasting God, vouchsafe, we beseech thee, to direct, sanctify, and govern, both our hearts and bodies, in the ways of thy laws, and in the works of thy commandments; that, through thy most mighty protection, both here and ever, we may be preserved in body and soul; through our Lord and Savior Jesus Christ.  Amen.

The 1979 Prayer Book reduced the entire Penitential Rite to the Kyrie only (in line with the Roman Rite) and even that was optional.  The present volume has restored the integrity of the Penitential Rite, making the Summary of the Law the standard text and offering the Decalogue as an option in its place, albeit without the former rubric requiring it monthly.

Here what our Lord Jesus Christ says:

You shall love the Lord your God with all your heart
and with all your soul and with all your mind.
This is the first and great commandment.
And the second is like it:
You shall love your neighbor as yourself.
On these two commandments depend all the Law and the Prophets.

Either on its own or (as in earlier Prayer Books) as a follow-up to the Decalogue, this Summary confronts the worshiper with the fundamental moral demand upon all Christians: to love God and neighbor with every aspect of one’s being.  Like as in the case of the Decalogue, this is a penitential moment: our unworthiness is called to mind, and we rightly respond with the Kyrie or the Trisagion.  This brief Penitential Rite does not, however, come to the worshiper as a new subject or focus within the liturgy, but actually serves as an answer to prayer: in the Collect for Purity we pray for the cleansing of our hearts by the inspiration of the Holy Spirit, which the Penitential Rite immediately addresses.  If we are to “perfectly love” God and “worthily magnify” his holy Name, we must confront ourselves with the Law of Moses, or Christ’s Summary of the Law, and cry out “Christ, have mercy upon us.”