How do we know that Saul was not a true believer?

Throughout the life of Saul, the evidence points toward him not being a true believer. Although he was anointed king and momentarily empowered by the Spirit to prophesy, his character and actions demonstrate a lack of spiritual regeneration. He was turned 'into another man', yet this 'another heart' was not synonymous with a new heart or true conversion. It simply indicated a change in role—from a farmer to king—but did not imply a transformation of faith leading to salvation. King Saul ultimately pursued self-interest over the welfare of his people and engaged in disobedience to God's commands, which aligns with the principle that genuine belief produces enduring faithfulness and fruitfulness.