"Complete sincerity" means being completely honest and genuine in your thoughts, words, and actions, without any hidden agendas or deceit. Full definition
It is one thing to tell a pollster, perhaps in complete sincerity, that family and personal moral values are our chief concerns. (firstthings.com)
How can we distinguish between deceit and the word of God when we may believe with complete sincerity that what we say is the truth? (religion-online.org)
And I tell you honestly, I prayed to Jesus in complete sincerity and with abject submission. (religion.blogs.cnn.com)