God's call on the Christian is not "free thinking" or "positive thinking" but holy thinking. Only God can change our minds so we think his thoughts after him.
“Finally, brothers, whatever is true, whatever is honorable, whatever is just, whatever is pure, whatever is lovely, whatever is commendable, if there is any excellence, if there is anything worthy of praise, think about these things. What you have learned and received and heard and seen in me—practice these things, and the God of peace will be with you.”
(Philippians 4:8–9 ESV)