The metaphysical problem involved here can no more be avoided than it can be solved. How can one personality enter another? The candid reply would be simply that we do not know, but a near approach to an understanding may be made by a simple analogy borrowed from the old devotional writers of several hundred years ago. We place a piece of iron in a fire and blow up the coals. At first we have two distinct substances, iron and fire. When we insert the iron in the fire, we achieve the penetration of the fire by the iron. Soon the fire begins to penetrate the iron and we have not only the iron in the fire but the fire in the iron as well. They are two distinct substances, but they have co-mingled and interpenetrated to a point where the two have become one.
In some such manner does the Holy Spirit penetrate our spirits. In the whole experience we remain our very selves. There is no destruction of substance. Each remains a separate being as before; the difference is that now the Spirit penetrates and fills our personalities and we are experientially one with God.
Verse
Guard the good deposit that was entrusted to you—guard it with the Holy Spirit who lives in us. 2 Timothy 1:14
Thought
In some such manner does the Holy Spirit penetrate our spirits.
Prayer
Penetrate our personalities, Spirit, change us from the inside out to make us one with the Father.