In verses 4 and 5, Peter gives a definition of the Christian life. To begin with, he says that a healthy Christian is continually coming to Jesus. This is how they are built up as living stones. Regarding the picture of stones, we must remember that at one time they were, spiritually, dead stones without any spiritual life. Jesus the cornerstone sent out his gospel to them and they became living stones because the Holy Spirit indwelt them as believers.
Further, because they are continuing to come to Jesus, it means that as stones in his temple they should be getting bigger, and enlarging together. The living stones are amorphous, with no defined or limited shape. Instead they are always growing in grace. Coming to Jesus as the fountain of grace results in spiritual growth.
There is also a sense in which Peter, since he is depicting the people of God as a temple, is describing their continual coming to Jesus as that of worshippers. They approach Jesus with a sense of veneration continually in order to offer spiritual sacrifices. Peter mixes his metaphors as it were and the living stones of the temple are also the priests in the temple. Unlike the priests in a literal temple, their offerings are not physical presentations. Instead they are spiritual activities. What do they include?
The first aspect is personal consecration to Jesus. Paul describes this spiritual activity in Romans 12:1-2. In light of all that God has done for us, we are to present our bodies as a living sacrifice. We are to take each part of our body and dedicate them to Jesus. What a difference there would be if we lived this out day by day!
The second aspect is verbal appreciation of Jesus. In the temple in Jerusalem, the impressive stones used in its construction never said a word. It is very sad when the living stones of the new temple imitate the dead stones of the old temple. The function of the living stones is to speak well of their Master and commend him as much as possible.
The third aspect is costly activities for Jesus. I suppose the clearest example of this is Mary of Bethany. Probably, it is not possible to do this all of the time. She certainly did not give her expensive ointment to Jesus every day. It is right to be devoted to Jesus, but it is also good sometimes to do something special for Jesus.
Yet even when the living stones function in this way and show great dedication to Jesus, the fact is that they are still imperfect and their spiritual activities are never accepted in themselves. There is only one way by which they can be accepted and that is through Jesus.
Peter reminds his readers also that their position is a great honour (v. 7). They are contrasted with those who stumble over Jesus because they do not think he is a suitable cornerstone. The destiny of such is to be ashamed of their choice, which destiny will be given to them at the Day of Judgement. It will be very different for those who belong to the spiritual temple. They are going to be the dwelling place of God, they are going to be the recipients of his blessings, they are going to share the fortunes of the Cornerstone. They are going to be glorified.