Some people lack faith. For some, they don't even know how to go about it. Being full of faith is a difficulty for them.
Say this prayer for such a person:
Gracious father in heaven, you are the God who was, who is and who is to come. You are capable of anything so I bring (person's name) to you, and I sincerely pray that you grant him(her) the grace to have faith. Help him(her) to find an unshakeable faith, grant him(her) the knowledge they need to boost the faith they find, and let them become a better person, Amen.