Believing in Yahweh...

 ... and not obeying Him is exactly what the devil does. 

An Undivided Heart

Among the gods, there is none like you, Lord; no deeds can compare with yours. 9 All the nations you have made will come and worship before you, Lord; they will bring glory to your name. 10 For you are great and do marvelous deeds; you alone are God. 11 Teach me your way, LORD, that I may rely on your faithfulness; give me an undivided heart, that I may fear your name. 12 I will praise you, Lord my God, with all my heart; I will glorify your name forever. 13 For great is your love toward me; you have delivered me from the depths, from the realm of the dead. Psalm 86:8–13

David asks for an “undivided heart” (verse 11). A divided heart can have many forms. There is the insincere heart, in which what is said out loud is not matched by the inner attitude (Psalm 12:1). There is the irresolute heart, which cannot fully commit itself (James 1:6–8). Even hearts regenerated by the Spirit and loving God retain much of their older willful resentment of his authority (Romans 7:15–25). David’s goal is not psychological healing for its own sake, but to “fear” God—to give him joyful, awe-filled love with his entire being. The way to this new heart is not introspection but deliberate worship (verse 12). “I will praise you,” he says.

Psalm 78:7-10

Psalm 78:7-10

7 so that they should set their hope in God and not forget the works of God, but keep his commandments; 8 and that they should not be like their fathers, a stubborn and rebellious generation,
a generation whose heart was not steadfast, whose spirit was not faithful to God. 9 The Ephraimites, armed with the bow, turned back on the day of battle.10 They did not keep God's covenant,but refused to walk according to his law.

This psalm recounts the history of Israel from its deliverance from Egypt to the kingship of David. Its negative lesson is that this history not be repeated in the lives of the listeners (verse 8). The positive lesson is that believers be marked by true faith (verse 7). We should not just know the truth about who God is (verse 7) but must trust him from the heart (verses 7 and 8) and show this saving faith through a changed life of obedience (verse 7). Throughout history, many have honored God with external behavior but failed to have converted hearts (Isaiah 29:13; Jeremiah 4:4). Are you just going through the motions of religion, or have you been born again (John 3:1–16)? 

The “men of Ephraim” are the northern tribes of Israel (verses 9–10) that fell into idolatry (1 Kings 12) and were deported and lost to history (2 Kings 17). The root of their problem was spiritual forgetting (verse 11). Christians too can stagnate because they “forget that they have been cleansed from their past sins” (2 Peter 1:9). The key is to have a heart constantly vitalized by deliberate remembering of the costly sacrifice of Jesus. We must remember that for our sins Jesus was, as it were, forgotten (“Why have you forsaken me?” Matthew 27:46) so that God can now no more forget us than a mother her nursing infant (Isaiah 49:14–16). Remembering that will make you a great heart. 

The Songs of Jesus: A Year of Daily Devotions in the Psalms - Keller (p. 184-5).