The readings from both Isaiah and Matthew are fairly clear, so I would like to focus on the psalm instead.  Individual lines here summarize and encapsulate elements of the spiritual life that we can lose sight of in the crowd of our concerns and the many and varied calls upon our attention.

"The Lord is my light and my salvation; whom should I fear?" 

This statement does not only apply to the end of my life and to the critical or painful events in that life, it touches even the smallest things.  God is taking care of us, constantly attentive to all our needs and desires: there is no such thing as coincidence, no real absurdity or pointlessness to our existence, only God's loving providence.

"One thing I ask of the Lord; this I seek: to dwell in the house of the Lord all the days of my life."

This is what Jesus promises at the Last Supper in John's Gospel when he speaks about there being many mansions in his Father's house and his preparing a place for us.  Our place is and always has been with the Father: as Augustine says in the opening pages of his Confessions, "You made us for yourself, O Lord, and our hearts are restless until they rest in you."

"I believe that I shall see the goodness of the Lord in the land of the living."

God made us for life and not for death, for love and not for fear.  This affirmation of life, whether in this world or in the next, is a continuation of the first verses.

What the lines from this psalm that we use in today's liturgy mean, at least to me, is that we are not only in God but that we should repose in him as trustingly as a child in its father's or mother's arms.  Repose: to be there and to be at ease there and to hope to remain there in peace, knowing that we are loved and cared for beyond our knowledge or hope.