To trust God you must trust His word.  Every word in the Word of God is pure.  When you know God's Word and have trust in that Word to the end that you act upon it, God becomes your shield.  But, when you know what God's Word says and do not act upon it, you have no shield because you do not trust God.

Genesis 14:18-23
    And Melchizedek king of Salem brought forth bread and wine: and he was the priest of the most high God.
    And he blessed him, and said, Blessed be Abram of the most high God, possessor of heaven and earth:
    And blessed be the most high God, which hath delivered thine enemies into thy hand. And he gave him tithes of all.
    And the king of Sodom said unto Abram, Give me the persons, and take the goods to thyself.
    And Abram said to the king of Sodom, I have lift up mine hand unto the Lord, the most high God, the possessor of heaven and earth,
    That I will not take from a thread even to a shoelatchet, and that I will not take any thing that is thine, lest thou shouldest say, I have made Abram rich:

Melchizedek blessed Abram and Abram gave Melchizedek, the priest of the most high God, tithes (a tenth) of all.  The tithe was (and is) the minimum response to the blessings God gives.  It shows trust in God as our provider.

The king of Sodom offered Abram great wealth as a reward for Abram's help.  In response Abram replied: "I have lift up mine hand unto the Lord."  This was an expression of complete surrender to God - "the possessor of heaven and earth."  Abram was saying that God owns everything in heaven and earth and out of His abundance He would provide for Abram.  Abram was not about to put his trust in the king of Sodom.  Abram's source of supply for his needs was God.

Abram declared that he would not receive even a shoelace from the king of Sodom "lest thou shouldest say, I have made Abram rich:"  God was the supplier of Abram's needs.  Abram gave his tithe to the man of God in recognition of God as his provider.  He also refused the support of the king of Sodom.  His total trust was in the most high God.

Genesis 15:1
    After these things the word of the Lord came unto Abram in a vision, saying, Fear not, Abram: I am thy shield, and thy exceeding great reward.

When we trust God's Word and carry it out God becomes our shield and our exceeding great reward.  Abram had demonstrated his trust in God by giving the tithe and refusing the support of the king of Sodom.  In response God told Abram that He, God, was Abram's shield and exceeding great reward.  Abram did not have to worry about having an abundance of all he needed. He also did not need insurance to replace damaged or stolen goods as God was going to be his protection.  God was his shield.

  David was a man of war.  David was also a man after God's own heart.  David trusted God by acting on God's Word.  When God delivered him unharmed from all his enemies David spoke and wrote these words which in military terms describe God.

God is my rock:  a rock being a high place of safety and refuge in a battle.  He is my fortress: a walled place of protection from the enemy.  He my deliverer.  He fights my battles for me.

When we trust God He becomes our shield.  When the enemy attacks God is our protection.  God is our shield.

In all of the science fiction writing and movies of our day aircraft and space ships have force fields and shields which repel all of the attacks of the enemy.  In the real world the metal in the airplane, tank, or ship protects the occupant from that which would attempt to attack it.  In the military battles of Biblical times round metal or wooden shields provided protection.  When we trust God He shields us.  God is a far better shield than the wooden shields of Biblical times or the metal bodies of tanks and aircraft today.  He is a far better shield than even the imagined force fields and electronic shields of the science fiction writers.

When you are down & need help - God lifts you up, makes you shine, and shields you from trouble.

Psalm 5:11-12
    But let all those that put their trust in thee rejoice: let them ever shout for joy, because thou defendest [covereth or protectest] them:  let them also that love thy name be joyful in thee.
    For thou, Lord, wilt bless the righteous; with favour wilt thou compass [crowneth] him as with a shield.

When we trust God He shields us with His divine favour - Grace!  His grace protects us like a shield held over the head protects the soldier from the falling arrows and spears of the enemy.

Psalm 28:7
    The Lord is my strength and my shield; my heart trusted in him, and I am helped: therefore my heart greatly rejoiceth; and with my song will I praise him.

When your heart trusts God He is your shield, your help, and your strength.  On your own you will never be strong enough, but with God as your strength you will be the victor every time.

Psalm 33:20-21
    Our soul waiteth for the Lord: he is our help and our shield.
    For our heart shall rejoice in him, because we have trusted in his holy name.

Ever hear "God helps them that help themselves?"  God helps them that trust in him as their shield and strength.  God helps those whose hearts rejoice in him and whose souls wait for his deliverance.  If you are going to "do it on your own" then you are going to really be on your own.  Trust God and He will be your shield and help.

Psalm 84:11-12
    For the Lord God is a sun and shield: the Lord will give grace and glory: no good thing will he withhold from them that walk uprightly.
    O Lord of hosts, blessed is the man that trusteth in thee.

Blessed is the man that trusteth God.  God is for that man a "sun and a shield."  A sun because the Lord will give that man glory - brightness! And a shield because He shields us with His grace.  He withholds no good thing from those that trust in Him.  Just as the tree planted by the river does not see the drought but continues to grow and yield fruit (Jeremiah 17:7-8), the man that trusts God is not affected by the negative attacks of the adversary because he is continually filled with good things by the grace of God.

Psalm 91:4
    He shall cover thee with his feathers, and under his wings shalt thou trust: his truth shall be thy shield and buckler.

When we trust God by acting on His Word, His truth becomes our shield and our buckler.  A buckler is a spike that sticks out from the center of a shield making it an offensive weapon as well as a defensive one.  God is our strength.  He is our deliverer. He is our shield and He fights for us.  It is interesting that in this verse the figures related to war and battle are coupled with a beautiful image of God as a mother hen that shelters her chicks under her wings.  He protects us just like that hen protects her young when we trust Him.  His truth is our shield and buckler.  You trust God by trusting His Word.

Psalm 119:114
    Thou art my hiding place and my shield: I hope in thy word.

Once again, we trust God by trusting His word.  God becomes for us a shield and "a hiding place" when we trust in Him.

Ephesians 6:16
    Above all, taking the shield of faith [believing], wherewith ye shall be able to quench all the fiery darts of the wicked.

The Greek word translated "shield" comes from the word used for "door."  This shield is like a tall oblong rock used to cover a cave entrance or the door of a house.  To believe is to have trust and confidence in the Word of God to the end that you act upon it.  When we believe God's Word He becomes for us a large shield (like a door) held over our heads able to quench all the fiery darts of the wicked.