“who trusts in humans.” This is not a blanket statement that we cannot trust anyone at any time. There are obviously times when it is important to trust people, but this is certainly a warning about trusting people. In the context of Jeremiah, the people had turned from God to idols and were exceedingly sinful (Jer. 17:1-2). Jeremiah’s prophetic ministry was to confront all Israel: the kings, officials, priests, and people (Jer. 1:8-19). There were not many people he could trust. In the same way, people today need to be diligent about who to trust. Many leaders and even many clergy, are wrong either out of ignorance or for their own gain.
Occasionally a Trinitarian will argue from Jeremiah 17:5 that Jesus cannot be a man because we are expected to trust Jesus, but not to trust men, and therefore Jesus must be a God-man (there are not many Trinitarians who make that argument, but some do). That analysis misses the point of this verse. The verse and its context must be read to get its proper meaning. The immediate context reveals that a person is cursed if he trusts man and also turns his heart away from the Lord. But we are not turning our hearts away from God by trusting in His Son Jesus. On the contrary, “he who does not honor the Son does not honor the Father” (John 5:23). God is the one who made Jesus our Lord and Head of the Church. Indeed, our hearts would be turning from the Lord if we did not trust Jesus. This same logic applies to other servants of God. The people were not cursed when they followed Moses, or Joshua, or David, and trusted in what they said, because these men were acting for God. Exodus 14:31 says the people trusted God and Moses. The husband of the virtuous woman is blessed when he trusts in his wife, as Proverbs 31:11 (KJV) says, “The heart of her husband safely trusts in her.” It is clear that there are times when trusting another person is completely appropriate.
[For more information on Jesus being the fully human Son of God and not being “God the Son,” see Appendix 10, “Jesus is the Son of God, Not God the Son.” For more on “the Holy Spirit” being one of the designations for God the Father and “the holy spirit” being the gift of God’s nature, see Appendix 11, “What is the Holy Spirit?”].
“and makes flesh his arm.” The word “arm” is a literal translation of the Hebrew, and was idiomatic for “strength” but the Hebrew idiom is clear enough to be left in the text. The “arm” of Yahweh is often used for the strength or might of Yahweh (cp. Exod. 6:6; 15:16; Ps. 79:11; 89:10; Isa. 33:2; 51:9; 53:1). Here in Jeremiah 17:5, the ungodly person makes “flesh,” i.e., people and what they say, his “arm,” his strength. But such a person will end up cursed and thus disadvantaged both in this life and the next.