# has given us understanding

"has enabled us to understand the truth"

# we are in him who is true

"we are united to him who is true by being united to his Son Jesus Christ" 

# him who is true

"the true one" or "the real God"

# This one is the true God

"This one" refers to Jesus Christ, or "This one" refers to the one true God.

# and eternal life

"and the one who gives eternal life"