Amongst Quakers, it's sometimes assumed that this phrase originates in early Quaker history. My understanding is that its first use was in the 1955 booklet of the same name published by American Friends Service Committee. It does reflect, however, the long tradition of Quakers challenging those in positions of power, coupled with an understanding that we all have access to an understanding of some aspect of a deeper, spirit-inspired, truth. What is intriguing now is how widely the phrase has come to be used completely outside the Quaker context, with many political, campaigning or civil society groups describing a range of activities as speaking truth to power. The truth in these contexts is most likely to be a firmly felt conviction about a particular issue, or even simply the factual but inconvenient truth that those protesting want to make sure those in power can't ignore.
There are now at least two variants that I have heard Quakers and Quaker organisations use. The first is 'Seeking truth with power'. This stems from two shifts in perspective from that assumed in the original phrase. Firstly, whilst we think we have some understanding of a truth, isn't it arrogant for us to assert that The Truth is fixed, and that once we have found it, our Quaker practice of seeking should be at an end? Secondly, in peace work it can be better to find approaches that recognise that the person we're seeking to influence has their own entirely valid experiences and understanding of truth. In speaking with them, might it be better to seek a truth that has meaning for both parties, if they're to be able to make the changes needed?
Finally, 'Speaking Truth with Power' another recent variant. Seeking change through nonviolent action of different sorts is a way of taking back power, which is also strengthened by action together in groups and communities, of faith and other common interests. If it's a matter of equalising a previously unequal and unjust relationship, becoming empowered so that the truths or concerns will be taken seriously. And the importance of nonviolent power is that it doesn't seek to be power over others, but to look to at how 'power with' can be the basis of a shared solution.
All of these will be important in the coming few years in the UK in the light of the very recent General Election. The Minute from our also-very-recent Yearly Meeting will, I imagine, be a source for encouragement for many - and not just for speaking in words but equally, if not more, in our actions.
Part of the long minute reads
"Our current political and (especially) economic systems only recognise and encourage part of the human condition, the selfish, competitive, greedy part. So much of what is good and beautiful and true in the world is being trashed. The model of power as domination needs to be challenged and replaced with a model of power as service to the community; in doing this, we need to live our testimony and hold firm to its source in faith. .... We must remember that what makes the real difference is not adding further to the words in the world but being and living out the new social order...."