Let us pray (in silence) [to God the source of all being, through the eternal Word, in the power of the Holy Spirit]
Almighty and everlasting God,
you have given to us your servants grace,
by the confession of a true faith,
to acknowledge the glory of the eternal Trinity,
and in the power of your divine Majesty to worship the Unity,
may we, through the steadfastness of this faith,
evermore be defended from all adversities;
through Jesus Christ
who is alive with you,
in the unity of the Holy Spirit,
one God, now and for ever.
The above is my reworking for my Book of Prayers in Common in which I seek to provide a set of collects with history and commentary.
Trinity Sunday is a Feast Day, hence this collect is not used during the week following – the collect on weekdays, this year, is that for Ordinary Week 10.
Trinity Sunday is notorious for, through attempted explanations of the Trinity, oversimplifying the doctrine to the point of affirming exactly what the doctrine is not trying to convey. Sermons regularly erroneously stray into tritheism/polytheism (that there are three gods); modalism (that there are three forms of God – like steam, liquid, and ice); and Subordinationalism (God the Father is the greatest. Coming in second is God the Son, followed by the second runner-up, the Holy Spirit).
Click on the following link to read my commentary and reflection on Trinity Sunday and its collect: Trinity Sunday.
The following may be another useful quote:
A world of one dimension would be a straight line. In a two-dimensional word, you still get straight lines, but many lines make one figure. In a three-dimensional world, you still get figures but many figures make one solid body. In other words, as you advance to more real and more complicated levels, you do not leave behind you the things you found on the simpler levels: you still have them, but combined in new ways–in ways you could not imagine if you knew only the simpler levels.
Now the Christian account of God involves just the same principle. The human level is a simple and rather empty level. On the human level one person is one being, and any two persons are two separate beings- just as in two dimensions (say on a flat sheet of paper) one square is one figure, and two squares are two separate figures. On the Divine level you still find personalities; but up there you find them combined in new ways which we, who do not live on that level, cannot imagine. In God’s dimension, so to speak, you find a being who is three Persons while remaining one Being, just as a cube is six squares while remaining one cube. [Mere Christianity, C.S. Lewis p. 137-138]
Further resources for Trinity Sunday may be found at Textweek.