Saint Francis of Sales was a popular French saint born in the 15th century France. Here are few of his verses on the simplicity.

Passage

Simplicity is nothing else but a pure and simple act of charity,
whose only end is the love of God. Our soul is truly simple when
we have only this end in all we do.

There is a certain simplicity of heart in which consists the perfection of all perfection.
It is when our soul, regarding God alone,
applies itself simply and with much fidelity to accomplish His rules,
and to follow the means of sanctification He has prescribed, without
desiring or undertaking any other.

True simplicity is like that of children, who think, speak, and act
openly, without malice. They believe all that is told them; they
have no care over themselves; they do not even think for themselves,
especially when with their parents; they keep close to them, hoping
all from them, knowing that they are loved.

Abbe Lasausse translated by James O’Brien. Short Meditations for Every Day of the Year. Benziger Brothers, 1890.