Read Psalms 28:7  In the first part of this verse, David says that the Lord is two things to him.  What are they?  A couple of days ago we talked about offense and defense.  When David went out against Goliath, what offensive weapon did he use?  When he chopped Goliath's head off, what offensive weapon did he use?  But to fight against Satan and his great power, we also need a defensive weapon.  David says in this verse that God is his shield.  Soldiers in David's time used a shield to defend themselves against their enemy's sword blows.  When a soldier saw a sword blow coming from his enemy, the wise soldier would hold onto his shield strongly and let the shield take the force of the blow.  But then immediately after the enemy's blow hit the shield, the soldier would strike at the enemy with his sword before the enemy could get his shield up in defense.  David goes on to say that his heart trusted in God and that he was helped because he was trusting in God.  It is a blessing to know that God is helping us.  Because David knew that God was helping him, he went on to say that his heart greatly rejoiced.  Rejoiced means to be happy.  David was so happy that he began to sing.  When we are sad, we go around all grumpy and frowny.  But when we are happy, when we rejoice, we have a song in our hearts and on our lips!  Isn't it much better to be happy and to rejoice in the Lord than to be afraid and upset and grumpy all day long?