From the beginning of the revelation of God to man, there have been those who have looked to be at variance with the essentials of the message. Even as Peter shared with those Jews and proselytes in Jerusalem, there were likely those who had set out to subvert parts of the message he brought for their own ends. Since the visible formation of the body of Christ until the present day, heresy has been present to confuse and provide a stumbling block for those seeking the truth.
The concept of child-like faith, in evangelical circles, seems to be taught in such a way that we should have child-like faith and not question too much. Instead, blind faith is sufficient. Just like a child does not know a lot but just blindly trust parents and those in authority, so we should not be too critical and instead just accept all that our pastors teach us.
Sanctification, in essence, is the process by which a person whose heart has been changed by the Holy Spirit is made more like Christ every day until His return or their promotion to heaven. The process is always the believers struggle with sin, empowered by the Holy Spirit. I have found John Owens Mortification of Sin to be a very good resource for understanding this. There is always going to be the conflict between the spirit and the flesh (Romans 7).