Collect for Trinity Sunday (BCP, p. 186)
Almighty and everlasting God, who hast given unto us thy servants grace, by the confession of a true faith, to acknowledge the glory of the eternal Trinity, and in the power of the Divine Majesty to worship the Unity; We beseech thee that thou wouldest keep us stedfast in this faith, and evermore defend us from all adversities, who livest and reignest, one God, world without end. Amen.
Our true faith as Christians is found in the recognition that we must believe in the redeeming work done by Jesus Christ on the cross. Taking that one step further though, we must also believe that the same person who became Man, took sin upon Himself, died so that we will never die, rose from the dead, and ascended to the right hand of the Father, is in fact God. Not some 'other.' Not something less than God, but rather, Jesus is God. The Spirit of God who dwells with us today is in the same manner God as the first and second persons are.
No words can adequately explain something that our human vocabulary is incapable of mastering. We can only do what Archbishop Cranmer asks in the first part of the petition - for true faith through grace. He does not ask God to give us a blueprint for the Trinity in terms that we can understand. He left the Trinity a mystery, and so should we. He left it in a way that only faith can do it justice.
I pray for that faith every day, and I believe in the Unity of the Trinity - leaving the full comprehension for the day when I can see and understand face-to-face!