# to those whom God has called

"to the people God calls"

# we preach Christ

"we teach about Christ" or "we tell all people about Christ"

# we preach Christ as the power and the wisdom of God

"we preach that God acted powerfully and wisely by sending Christ to die for us" 

# the power ... of God

Another possible meaning is that Christ is powerful and it is through Christ that God saves us.

# the wisdom of God

Another possible meaning is that God shows the content of his wisdom through Christ.