Radak: It is a result. If he trusts in Hashem, Hashem will be his security; he will not fear any evil.

Malbim: It is a condition. It is not enough to trust in Hashem to save him via natural means. Then, his trust is not in Hashem, rather, the natural means via which he hopes Hashem will save him. Granted, he is not cursed, for he trusts in Hashem, but he is not blessed. He is blessed only if he trusts in Hashem to save him without any intermediary at all. Hashem is his Mivtach, i.e. the matter via which he will attain his trust.