Faith is a gift – or so the Bible says in Ephesians 2:8 – and faith is substance and assurance (Hebrews 11:1). So those with faith might not require much else. They believe in Jesus having, like me, that assurance in their hearts where they can hear and feel the Holy Spirit. The other side of that though is that they can be deceived. So God has put in place two other things to help, the Scripture and the Church. So we have three things which keep us on the straight and narrow so to speak.
Here is a check for the church or group you are in: Do they honour the personal in dwelling Spirit of God in you and the Scripture? Some big groups do not: churches with a clergy say you cannot know God safely unless you are guided by clergy and some would rather you did not read the Scripture at all but let it be read and interpreted to you through the same clergy. Sound familiar? So they remove two safeguards and leave you with only one. And what if that turns out to be corrupt?