I have checked a number of Bible translations and only the New Living Translation has "This is the only work." "Only" is not to be found in any of the other translations. Indeed, the Greek is Τοῦτό ἐστιν τὸ ἔργον τοῦ θεοῦ ἵνα πιστεύητε εἰς ὃν ἀπέστειλεν ἐκεῖνος / "This is the work of God, that you should believe in Him whom He sent." So who is adding to the Word of God here? And if we are to believe in Jesus, then we must also believe in his instruction that we must keep the Commandments, not merely believe in the his salvific work.
Joh 6:29 Jesus told them, This is the only work God wants from you: Believe in the One He has sent.
If you want to go with this that is good also. It says faith in Jesus is the work of God, not our own ability. But it is still the ONE thing that is needed.
(MKJV) Jesus answered and said to them, This is the work of God, that you believe on Him whom He has sent.