A "Believer" (a person who has been BORN AGAIN of the Holy Spirit, and, by FAITH, cleansed of their SIN) can easily become part of a Denominational / Doctrinal / theological system that encompasses both TRUTH and HERESY. I'm part of one myself (the Assemblies of God Denomination)!!! However I've learned to "Eat the meat, and spit out the bones", by virtue of familiarity with the Bible, and what it teaches. Christianity isn't about "Doctrine", or "Theology", but about living FAITH and relationship with God. Romans 14 gives some insight closing with: "Whatever's not of FAITH is SIN".
Some denominations are more fully in accordance with the Biblical record, and some denominations are riddled with Paganism, and false doctrine / practices.
"KNOWINGLY" is the issue. it's rather a contradiction. A Christian would find it difficult to impossible to ACTUALLY BELIEVE anything that they KNOW to be false!!!