O Israel, trust thou in the Lord : he is their help and their shield. O house of Aaron, trust in the Lord : he is their help and their shield. Ye that fear the Lord , trust in the Lord : he is their help and their shield.
Psalms 115:9-11 KJV
Three times God gives the instruction to Israel to trust in the Lord. Why? Because He (God) is their help and their shield. Spurgeon tells us that this psalm was sung at the Passover as it is part of The Hallel. The three-fold reminder to trust in the Lord, then, is most likely a direct reference to the fact that God proved Himself as a help in parting the Red Sea and a shield in using the pillar of cloud and fire to prevent the Egyptians from prevailing over them.
We would do well to remember our Lord’s deliverance from our sins as proof of His help (in that he redeemed us) and His shielding protection (in that He keeps us in the palm of His hand and none is able to pluck us from it). O that these truths would drive us to trust in the Lord and not in ourselves or vain idols!