Heb 12:1  “Therefore, since we are surrounded by so great a cloud of witnesses, let us also lay aside every weight, and sin which clings so closely, and let us run with endurance the race that is set before us,.”

John Gill’s Commentary:

with so great a cloud of witnesses; or “martyrs”, as the Old Testament saints, the instances of whose faith and patience are produced in the preceding chapter: these, some of them, were martyrs in the sense in which that word is commonly used; they suffered in the cause, and for the sake of true religion; and they all bore a noble testimony of God, and for him; and they received a testimony from him; and will be hereafter witnesses for, or against us, to whom they are examples of the above graces: and these may be compared to a “cloud”, for the comfortable and reviving doctrines which they dropped; and for their refreshing examples in the heat of persecution; and for their guidance and direction in the ways of God; and more especially for their number, being like a thick cloud, and so many, that they compass about on every side, and are instructive every way. Hence the following things are inferred and urged,

Vincent’s Word Studies:

Witnesses (μαρτύρων) does not mean spectators, but those who have born 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 born witness to the truth, have entered into their heavenly rests 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. How striking the contrast of this conception with that of Kaulbach’s familiar “Battle of the Huns,” in which the slain warriors are depicted rising from the field and renewing the fight in the upper air with aggravated fury.

Barnes:

Seeing we also are encompassed about with so great a cloud of witnesses – The apostle represents those to whom he had referred in the previous chapter, as looking on to witness the efforts which Christians make, and the manner in which they live. There is allusion here, doubtless, to the ancient games. A great multitude of spectators usually occupied the circular seats in the amphitheater, from which they could easily behold the combatants; see the notes on 1Co_9:24-27. In like manner, the apostle represents Christians as encompassed with the multitude of worthies to whom he had referred in the previous chapter. It cannot be fairly inferred from this that he means to say that all those ancient worthies were actually looking at the conduct of Christians, and saw their conflicts. It is a figurative representation, such as is common, and means that we ought to act as if they were in sight, and cheered us on. How far the spirits of the just who are departed from this world are permitted to behold what is done on earth – if at all – is not revealed in the Scriptures. The phrase, “a cloud of witnesses,” means many witnesses, or a number so great that they seem to be a cloud. The comparison of a multitude of persons to a cloud is common in the classic writers; see Homer II. 4:274, 23:133; Statius 1:340, and other instances adduced in Wetstein, in loc.; compare notes on 1Th_4:17.

Jamieson, Fausset and Brown:

The image is from a “race,” an image common even in Palestine from the time of the Greco-Macedonian empire, which introduced such Greek usages as national games. The “witnesses” answer to the spectators pressing round to see the competitors in their contest for the prize (Phi_3:14). Those “witnessed of” (Greek, Heb_11:5, Heb_11:39) become in their turn “witnesses” in a twofold way: (1) attesting by their own case the faithfulness of God to His people [Alford] (Heb_6:12), some of them martyrs in the modern sense; (2) witnessing our struggle of faith; however, this second sense of “witnesses,” though agreeing with the image here if it is to be pressed, is not positively, unequivocally, and directly sustained by Scripture.

The “witnesses” in “the great cloud” are specifically those Paul specifically referred to in Chapter 11. Since the implications of “great cloud” can allude to a great number, some commentators tell us that the “witnesses” include all of those who have died in the faith and, in the sense of a sports stadium, “watch” us running our race, and “cheer” us on.

There is absolutely no scriptural support for the thought that we are to pray to dead “saints”, or that we must go through/with them to the throne of grace. Neither are the “prayers of the saints” mentioned in Rev 5 and 8 prayers of dead believers. rather they refer to prayers of living saints here on earth, mo matter what the time in history.