A.T. Robertson Commentary 1 John 2:29

"If ye know that he is righteous, ye know that every one also that doeth righteousness is begotten of him." — 1 John 2:29 (ASV)

If ye know (εαν ειδητε). Third-class condition again with εαν and second perfect active subjunctive of οιδα. If ye know by intuitive or absolute knowledge that Christ (because of verse 28) is righteous, then "ye know" or "know ye" (γινωσκετε either indicative or imperative) by experimental knowledge (so γινωσκω means in contrast with οιδα).

Is begotten (γεγεννητα). Perfect passive indicative of γενναω, stands begotten, the second birth (regeneration) of Joh 3:3-8.

Of him (εξ αυτου). Plainly "of God" in verse 9 and so apparently here in spite of δικαιος referring to Christ. Doing righteousness is proof of the new birth.