What do you envision at the word "shield"? I envision a metal or wooden disc shaped orb that soldiers would lift in battle to protect themselves from arrows shot at them by their enemies. A shield also held the Liege's crest, which was a way of identifying what clan the user belonged to. But is this what Genesis 15:1 is talking about? Being shielded physically? That would make sense at first, but further thoughts remind me that the physical shields did not always protect all the soldiers from harm. Arrows might penetrate the shield, or perhaps the shield was not big enough to cover their entire body, so arms or legs might take an arrow. Perhaps the person using the shield did not hold it in the correct position, allowing arrows to meet their intended mark.
And let's face it, in this sin sick world, Christians are not always protected physically from things that would hurt us. Sickness, accidents, violence, any number of things happen to good people all the time. So while God's shield may or may not be a physical shielding (it could be), there has to be more that He is telling us in this verse.
One thought that came out in our Sabbath School class was that the shield of God was a way of protecting us from our sinful selves. That is a comforting thought! Because me, myself, & I can often be my own worst enemy.
There is another thought that came to me as we were discussing the subject: Much like Jesus' robe of righteousness covering us during judgement, God's shield will be an impenetrable force of Jesus' spiritual protection that satan's accusations bounce off of. When we give our lives to God and accept Him into our lives, He covers us with Jesus' robe of righteousness and with Jesus' shield of spiritual protection. It is not our sinful selves that God/other people/satan see, it is Jesus' character living in us that they see. His shield is big enough to cover our whole selves, and strong enough that our enemy cannot penetrate. When we accept Jesus, we accept His shield, and as long as we choose to stay behind His shield, we cannot lose His protection that will carry us through this world of woe to an everlasting life with Him in Heaven.
God specifically identifies Himself in this verse. I AM, a name that pops up in both the old and new testament books identifying Himself as God. 'I am your shield.' Not I was your shield, not I will be your shield, but I AM your shield. If you believe in an all powerful God, if you allow Him to be your shield, you are protected indeed!