# these things

This refers to faith, virtue, knowledge, self-control, endurance, godliness, brotherly affection, and love, which Peter mentioned in the previous verses.

# they will keep you from being barren or unfruitful

"because you have them you will not be barren or unproductive" or "they will enable you to do much valuable work" 

# in the knowledge of our Lord Jesus Christ

"through your knowing God and Jesus our Lord"