Adam Clarke on James 1:18

Of his own will begat he us - God's will here is opposed to the lust of man, Jam 1:15; his truth, the means of human salvation, to the sinful means referred to in the above verse; and the new creatures, to the sin conceived and brought forth, as above. As the will of God is essentially good, all its productions must be good also; as it is infinitely pure, all its productions must be holy. The word or doctrine of truth, what St. Paul calls the word of the truth of the Gospel, Col 1:5, is the means which God uses to convert souls.

A kind of first fruits - By creatures we are here to understand the Gentiles, and by first fruits the Jews, to whom the Gospel was first sent; and those of them that believed were the first fruits of that astonishing harvest which God has since reaped over the whole Gentile world. See the notes on Rom 8:19, etc. There is a remarkable saying in Philo on this subject, De Allegoris, lib. ii. p. 101: God begat Isaac, for he is the father of the perfect nature, σπειρων εν ταις ψυχαις, sowing seed in souls, and begetting happiness.