Adam Clarke on Hebrews 13:22

Suffer the word of exhortation - Bear the word or doctrine of this exhortation. This seems to be an epithet of this whole epistle: and as the apostle had in it shown the insufficiency of the Levitical system to atone for sin and save the soul; and had proved that it was the design of God that it should be abolished; and had proved also that it was now abolished by the coming of Christ, whom he had shown to be a greater priest than Aaron, higher than all the angels, the only Son of God as to his human nature, and the Creator, Governor, and Judge of all; and that their city was shortly to be destroyed; he might suppose that they would feel prejudiced against him, and thus lose the benefit of his kind intentions toward them; therefore he entreats them to bear the exhortation which, notwithstanding the great extent of the subject, he had included in a short compass.

I have written a letter unto you in few words - Perhaps it would be better to translate δια βραχεων επεστειλα ὑμιν, I have written to you briefly, as επιστελλειν often signifies simply to write, and this appears to be its meaning here.