Humans, of course, are never simple. Any emotion we feel, any characteristic that describes us, is mixed with something else. Our hearts are divided and we are always double-minded. But even in our complicated state, our intention or goal can still be the simplicity of God. That seems to me to be the point of the prayer prayed in the first verse of the Ninth Psalm:
I will give thanks to the Lord with my whole heart;
I will tell of all your wonderful deeds.
As long as my mortal life continues, evil and injustice will always lurk within me, and at times even come to the fore. And yet I can single-mindedly and wholeheartedly proclaim to the world what I know about God's pure love, and thus partake of the simplicity of God.
My praise is most acceptable to you, O God, and so it is the greatest blessing I can attain to. Help me so to live in that spirit of praise that my life can be lived according to your will and that my thanks will themselves be a witness to your pure love and unalloyed goodness. In the Name of Jesus Christ, who taught me to pray: Our Father...