Witnesses (μαρτύρων) does not mean spectators, but those who have borne witness to the truth, as those enumerated in ch. 11.

Yet the idea of spectators is implied, and is really the principal idea. The writer’s picture is that of an arena in which the Christians whom he addresses are contending in a race, while the vast host of the heroes of faith who, after having borne witness to the truth, have entered into their heavenly rest, watches the contest from the encircling tiers of the arena, compassing and overhanging it like a cloud, filled with lively interest and sympathy, and lending heavenly aid.

Weight (ὄγκον). N. T.o, oLXX. Lit. bulk, mass. Often in Class. Sometimes metaphorically of a person, dignity, importance, pretension

loftiness, majesty, impressiveness. Rend. “encumbrance,” according to the figure of the racer who puts away everything which may hinder his running. So the readers are exhorted to lay aside every worldly hindrance or embarrassment to their Christian career.

And the sin which doth so easily beset (καὶ τὴν εὐπερίστατον ἁμαρτίαν)

of a sin which readily or easily encircles and entangles the Christian runner, like a long, loose robe clinging to his limbs. Beset is a good rendering, meaning to surround.

The sin may be any evil propensity. The sin of unbelief naturally suggests itself here.

With patience (διʼ ὑπομονῆς). Ὑπομονὴ includes both passive endurance and active persistence

The race (τὸν ἀγῶνα). Instead of a specific word for race (δρόμος), the general term contest is used

Having presented a long catalogue of witnesses under the old covenant, he now presents Jesus, the mediator of the new covenant, and the supreme witness

Christ is the leader or captain of faith, in that he is the perfecter of faith. In himself he furnished the perfect development, the supreme example of faith, and in virtue of this he is the leader of the whole believing host in all time

For the joy that was set before him

The joy was the full, divine beatitude of his preincarnate life in the bosom of the Father; the glory which he had with God before the world was. In exchange for this he accepted the cross and the shame

The contrast is designed between the struggle which, for the present, is alone set before the readers (ver. 1), and the joy which was already present to Christ. The heroic character of his faith appears in his renouncing a joy already in possession in exchange for shame and death

For consider (ἀναλογίσασθε γὰρ). Γὰρ for introduces the reason for the exhortation to look unto Jesus. Look unto him, for a comparison with him will show you how much more he had to endure than you have.