Why did Solomon go astray?

To understand Solomon's failures, consider widening the scope of your thinking. We can compare any person to Jesus and see flaws in that person. What may be instructive is to compare three kings who are not Jesus: Saul, David and Solomon. Each of these kings excelled in one area and had deficiencies in others. God is infinite, so once cannot assemble a perfect king by combining those three finite men. Nevertheless, by comparing their strengths and weaknesses, we may discern the features of a complete and godly leader.

I have a simplified model of the Trinity as it relates to the distinct spheres of a person: mind, soul, and body.

Jesus is the Word, hence represents intellectual ability, speech, and logic.

The Holy Spirit is the heart and soul of a person, comprising the emotions and desires.

The Father is the body of a person, disciplining our habits, managing time and performing physical miracles that manipulate matter.

Saul. With this model, we see that King Saul possessed excellency of the body: strong, tall, handsome, courageous and a charismatic leader. His lack of heart for the Lord was his downfall, and he was assailed by mental illness, which weakened his mind. For an excellent study in Saul's virtues, see http://www.scielo.org.za/scielo.php?script=sci_arttext&pid=S1015-87582022000300019#:~:text=At%20the%20same%20time%2C%20in,1%20Samuel%209%3A17).

David. King David is always described as being a man after God's own heart. He wrote many Psalms, which address our desires and emotions. They help us meditate on the Law and get its words into our heart so they may grow. David also had charisma and strength and courage, but made some terrible decisions, especially regarding his son Absalom. When he should have been off fighting a war and leading his army, which Saul would have been great at, he remained at home, saw Bathsheba, and fell into grievous sin.

Solomon. King Solomon is not noted as being a great warrior like David or Saul, and his heart strayed from God when he took foreign women as his wives. His excellence was in wisdom and intellectual ability.

Thus we can see that each of these three kings excelled in one of the three areas and was deficient in the other two. Only Christ has perfect knowledge (as one who is greater than Solomon, as Matthew tells us), an undivided heart, and is perfect in obedience (as evidenced by his testing in the desert and submitting to be crucified).

The Queen of the South will rise at the judgment with this generation and condemn it; for she came from the ends of the earth to listen to Solomon’s wisdom, and now something greater than Solomon is here. (Matthew 12:42)

This only partly answers the question, because many people with lesser abilities than Solomon proved more faithful to God. The conclusion to Ecclesiastes has these words: "Fear God and keep His commandments." The man was able to prove this dictum logically, but to follow those words wholeheartedly? That is a matter tied up in the secret places of his heart.

I know that God has given me many blessings, yet I take them for granted, lose my gratitude and slip into sin. It is easy for me to compare myself to Solomon and see how I am like him in my frailty. What amazes me is how Jesus remained true. Do not be amazed at Saul's failure, but be warned.

We all have deficiencies. If we rely on the Lord, we can overcome them, letting Christ bolster us in our areas of weakness, or getting help from the church. These kings did sometimes lean on God to overshadow their weaknesses, but they were not consistent.

God’s plan? Yes!

Was Solomon’s failure part of God’s plan? Yes. The basis for this is the Word of God. The Word of God is perfect and eternal. Jesus is the embodiment of that Word, as revealed in John 1. Since Ecclesiastes, Proverbs, Song of Songs and the passages in Kings and Chronicles that describe Solomon’s life are part of the Word of God, they are certainly part of God’s plan, hence also the record of Solomon’s moral failures. Indeed, Ecclesiastes is relentlessly focused on planning and time management. Solomon’s life was planned by God to teach us about how to plan our lives according to God’s timing and wisdom.

(For more on Solomon, I refer you to my book, Peace, like Solomon Never Knew. It is a lengthy commentary on Ecclesiastes, with additional notes on Proverbs and Song of Songs.)