“His heart will thus be firmly secured in God, who completes for us.” — Tanya]
This is not just a phrase.
Note that the author is not saying simply to trust in God but in “God who completes for us.”
It means that in whatever concerns service of God, one is not to rely on divine help (compare the Talmudic saying, “Everything is in the hands of heaven except fear of heaven”.
One may look to God for help and illumination, but one may not expect God to do the job for us.
We can expect, at most, that He will “complete for us”—if we begin.
-—Rabbi Adin Steinsaltz