How spiritual downfall beings:

They did not deviate from the king’s commands to the priests or to the Levites in any matter, including that of the treasuries. – 2 CHRONICLES 8.15

Although Solomon carefully followed God’s instructions for building the temple and offering sacrifices (8.13), he paid no attention to what God said about marrying pagan women. His sin in marrying a foreign wife (8.11) began his slide away from God.  No matter how good or spiritual we are in most areas of life, one unsurrendered area can begin a downfall.  Guard carefully every area of your life, especially your relationships.  Don’t give sin any foothold.