Question 3 of The Catechism of Christian Doctrine (The Penny Catechism) states that:
“God made me to His own image and likeness”
Monsignor Gilbey in his commentary, We Believe, comments that this means man has both intellect and will. The monsignor continues to indicate that while faith is reasonable it is also (if not primarily) an act of the will:
“People often think that becoming Catholic entrails acceptance of, let’s say, twenty-five successive propositions. No, becoming Catholic entails the acceptance of only one: namely, that the Catholic Church is God’s revelation. With that acceptance comes the consequent acceptance of the whole of the Church’s teaching in one single act.” (We Believe, Chapter 2, page 25)
A man’s will can be affected by the environment, his natural disposition or his current emotional state. The will can be volatile.
If the will is required to make a declaration of faith, how can anyone be certain that his declaration will not change tomorrow? How would someone know that they can say with certainty that they believe?