Can a believer deviate and fall?

St. Clement of Alexandria says, “A true gnostic [one who truly knows God … chooses faith or rejects it, with his full freedom.”⁸³ And St. Augustine says, “Wretched, then, must be any people that is divorced from this God … The City of God, however, has a peace of its own, namely, peace with God in this world by faith and in the world to come by vision.”⁸⁴