James, the Lord’s brother (Ga 1:19), identifies a certain kind of rejoicing as evil (Ja 1:16): boasting in our own plans as though they’re God’s is missing the mark. Yet God isn’t making up a new standard here: He’s not embellishing or extending His moral Law; it’s always been this way.
Since all sin is a violation of Torah, and any violation of Torah is sin (1Jn 3:4), this evil rejoicing must be rooted in some deviation from Torah. What law of Torah is violated by evil rejoicing?
Such rejoicing might be rooted in pride, in thinking of ourselves more highly than we ought (Ro 12:3), but then we must find some law forbidding pride, which is yet another challenge.
Perhaps loving our neighbors as ourselves (Le 19:18b) precludes pride, but even if we’re able to do so, could we not still rejoice in our boastings?
The evil in this boasting noted by James isn’t necessarily related to confidence in one’s abilities (1Co 7:7), but in failing to acknowledge our ultimate dependence on God. (Ja 4:15) How can we presume the power to succeed in our schemes if we can’t even keep our hearts beating or our brains from shutting down? (Lk 12:16-20) It’s like making a promise or a vow we can’t necessarily keep; there are limits to our personal capability and power, and we should always make promises with these limitations in mind.
It is a violation of Torah to make a vow to Jehovah and then not keep it. (De 23:21-23). When making promises or vows to God (or others) we should conform our speech to reality, acknowledging both our internal inclinations, weaknesses and disposition as well as our external limitations and dependencies.
This being true, it naturally follows we ought to employ the same humility and caution both in the context of planning and also in evaluating the likelihood of our success. (Pr 27:1)
Further, this implies God will not generally reveal how our plans are going to turn out (1Ti 3:14-15); He does do this at times (1Sa 30:8), but it is the exception rather than the rule. (Pr 16:9)
Torah doesn’t forbid appropriate confidence in our abilities, given us by God (2Co 11:6); it does encourage us to glory only in Him (Ps 62:7) and what He’s done for us. (Ga 6:14)
Boasting in our own plans as though we’re independent of God, without being mindful of Him, may also be a violation of the first and great commandment, to love God with all of our heart, soul, strength and mind. In some sense, every sin will violate this first commandment.