A.T. Robertson Commentary 1 John 5:3

"For this is the love of God, that we keep his commandments: and his commandments are not grievous." — 1 John 5:3 (ASV)

This (αυτη)

--that (ινα). Explanatory use of ινα with αυτη, as in Joh 17:3, to show what "the love of God" (4:9,12) in the objective sense is, not mere declamatory boasting (4:20), but obedience to God's commands, "that we keep on keeping (present active subjunctive as in 2:3) his commandments." This is the supreme test.

Are not grievous (βαρεια ουκ εισιν). "Not heavy," the adjective in Mt 23:4 with φορτια (burdens), with λυπο (wolves) in Ac 20:29, of Paul's letters in 2 Corinthians 10:10, of the charges against Paul in Ac 25:7. Love for God lightens his commands.