In his saints, i.e. in his angels, as appears by comparing Job 4:18, who are called his saints or holy ones, Deuteronomy 33:2 Psalms 103:20 Daniel 4:13,23 Mt 18:10 24:36; who though they were created holy, yet he could not be confident in them, nor they be confident in themselves that they would continue in their integrity if they were left to themselves, and not upheld by God's special grace and assistance. See Poole on "Job 4:18". The heavens, Heb. and the heavens, i.e. either,
1. The heavens properly so called; which though they be free from those drossy mixtures which are and appear in heavenly bodies, yet are not absolutely pure, but have some spots and blemishes in them; as philosophers have discovered, and the all-seeing God knoweth. Compare Job 25:5, where the stars are said not to be pure; unless the stars also there be metaphorically put for the angels, as they are Job 38:7, and for other holy ministers of God, as Daniel 8:10 Revelation 1:16,20 Revelation 12:1,4.
2. The angels that dwell in heaven; heaven being oft put for its inhabitants; either for God, as Psalms 73:9 Daniel 4:26 Luke 15:18,21; or for the angels that dwell in heaven, as Psalms 89:5, Psalms 148:1,2. So this is a repetition of the same thing in other words. And these are not pure, to wit, simply and perfectly, and comparatively to God; in which and such like respects God only is said to be good, and wise, and immortal. The angels are pure from corruption, but not from imperfection, nor from a possibility of sinning, if God should withdraw his help from them.