Why do you call God's Word an assumption? You sound like an agnostic/atheist: "I do not know therefore you cannot know."Everyone assumes their "divine assurance" is based on God's word.
That includes you.
Romans 8 v 15-16 You did not receive a spirit of slavery that returns you to fear, but you received the Spirit of sonship, by whom we cry, “Abba! Father!” The Spirit Himself testifies with our spirit that we are God’s children.