And it came to pass in those days - The designation of the time here is very general. It means "about" the time when the events occurred which had been just narrated.
He went out into a mountain - Jesus was accustomed to resort to such places to hold communion with God, Mark 6:46. He did it because it was retired, free from interruption, and fitted by impressiveness and grandeur to raise the thoughts to the God that had formed the high hills and the deep-shaded groves.
And continued all night in prayer to God - There has been a difference of opinion about this passage, whether it means that he spent the night in the act of "praying" to God, or in a "place" of prayer. The Jews had places of prayer, called "oratories," built out of their cities or towns, where they could retire from the bustle of a city and hold communion with God. They were built on the banks of rivers (compare Acts 16:13), in groves, or on hills. They were rude inclosures, made by building a rough wall of stone around a level piece of ground, and capable of accommodating a small number who might resort thither to pray. But the more probable opinion is that he spent the whole night in supplication; for:
1. This is the obvious meaning of the passage.
2. The object for which he went out was "to pray."
3. It was an occasion of great importance. He was about to send out his apostles - to lay the foundation of his religion - and he therefore set apart this time especially to seek the divine blessing.
4. It was no unusual thing for Jesus to spend much time in prayer, and we are not to wonder that he passed an entire night in supplication. If it be asked why Jesus should pray "at all" if he was divine, it may be replied that he was also a "man" - a man subject to the same sufferings as others, and, "as a man," needing the divine blessing. There was no more inconsistency in his "praying" than there was in his "eating." Both were "means" employed for an end, and both were equally consistent with his being divine. But Jesus was also "Mediator," and as such it was proper to seek the divine direction and blessing. In "this" case he has set us an example that we should follow. In great emergencies, when we have important duties, or are about to encounter special difficulties, we should seek the divine blessing and direction by "prayer." We should set apart an unusual portion of time for supplication. Nay, if we pass the "whole night" in prayer, it should not be charged as enthusiasm. Our Saviour did it. Men of the world often pass whole nights in plans of gain or in dissipation, and shall it be esteemed strange that Christians should spend an equal portion of time in the far more important business of religion?

In prayer to God - Or, in the prayer of God: or, in the oratory of God, εν τῃ προσευχῃ του Θεου. So this passage is translated by many critics; for which Dr. Whitby gives the following reasons: As the mountain of God, Exodus 3:1; Exodus 4:27; the bread of God, Leviticus 21:17; the lamp of God, 1-Samuel 3:3; the vessels of God, 1-Chronicles 22:19; the altar of God, Psalm 43:4; the sacrifices of God, Psalm 51:17; the gifts of God, Luke 21:4; the ministers of God, 2-Corinthians 6:4; the tabernacle of God, 2-Chronicles 1:3; the temple of God, Matthew 21:12; the synagogues of God, Psalm 74:8; are all things consecrated or appropriated to God's service; so προσευχη του Θεου must, in all reason, be a house of prayer to God; whence it is called τοπος προσευχης, a place of prayer, 1 Maccabees 3:46; and so the word is certainly used Acts 16:13; and by Philo, in his oration against Flaccus, where he complains that αἱ προσευχαι, their houses for prayer were pulled down, and there was no place left in which they might worship God, or pray for Caesar; and by Josephus, who says the multitude was gathered εις την προσευχην, into the house of prayer: and so Juvenal, Sat. iii. v. 296, speaks to the mendicant Jew: -
Ede ubi consistas; in qua te quaero proseucha?
In what house of prayer may I find thee begging?
See on Acts 16:13 (note). But on this it may be observed, that as the mountains of God, the wind of God, the hail of God, the trees of God, etc., mean very high mountains, a very strong wind, great and terrible hail, very tall trees, etc., so προσευχη του Θεου, here, may be very properly translated the prayer of God; i.e. very fervent and earnest prayer; and though διανυκτερευων may signify, to lodge in a place for a night, yet there are various places in the best Greek writers in which it is used, not to signify a place, but to pass the night in a particular state. So Appian, Bell. Pun. Εν τοις ὁπλοις διενυκτερευϚε μεθ' ἁπαντων - He passed the night under arms with them all. Idem, Bell. Civ. lib. v. διενυκτερευον - They passed the night without food, without any regard to the body, and in the want of all things. See more examples in Kypke, who concludes by translating the passage thus: He passed the night without sleep in prayers to God. Some of the Jews imagine that God himself prays; and this is one of his petitions: Let it be my good pleasure, that my mercy overcome my wrath. See more in Lightfoot.

(3) And it came to pass in those days, that he went out into a mountain to pray, and continued all night in prayer to God.
(3) In using earnest and long prayer in choosing twelve of his own company to the office of the apostleship, Christ shows how religiously we ought to behave ourselves in the choice of ecclesiastical persons.

And it came to pass in those days,.... When Christ was teaching by the lake of Gennesaret, or in one or other of the cities of Galilee near that place:
that he went out; of the synagogue and city where he had been:
into a mountain to pray; for the sake of solitude, and which lay near the sea of Tiberias; See Gill on Matthew 14:23.
and continued all night in prayer to God; or "with" God, as the Ethiopic version renders it; or "in the prayer of God" as the phrase may be literally rendered; not in a prayer of God's making; though the Jews (m) sometimes speak of the prayer of God, and give us a form of it: but either this respects the object of his prayer; it was made to God, as our translation suggests; or the nature, matter, and manner of it: it was a divine prayer, it regarded divine things, and was put up in a very fervent manner, and with great vehemence; so the coals of love or jealousy are said to be "coals of fire, which hath , the flame of Jehovah"; that is as we render it, "a most vehement flame", Song 8:6 In like manner, "prayer of God" is a most vehement prayer; strong cries sent up to God with great eagerness and importunity, fervency, and devotion; and such was Christ's prayer, and in which he continued all night: unless by the prayer of God should be meant, as is thought by many, an house of prayer to God, in which Christ lodged all night, and spent it in prayer to God in it. Certain it is, the Jews had their "proseuchre", or prayer houses. Philo the Jew (n) often speaks of them, and so does Josephus (o); and there seems to be mention made of them in the Talmudic writings: when R. Jochanan ben Zaccai came to Vespasian, in his camp before Jerusalem, Vespasian asked him, what he should give him? he replied (p),
"I desire nothing of thee but this "Jabneh", (a famous university,) that I may teach in it the disciples, and fix in it "an oratory", or "prayer house", and do in it, all the commandments said in the law.''
And in another place (q),
"R. Judah says, that Samuel said it is free for a man to make water within four cubits, , which I should choose to render, "of the proseucha", or "prayer house":''
though the Gemarists afterwards, and so the gloss seem to explain it of the time after prayer, in which a man should wait before he evacuates, even as long as he might go the length of four cubits. Juvenal (r) has reference to one of these oratories, when he says, "in qua te qucero proseucha?" and in one of these, it is very likely, Christ was in prayer all night long; for by the sea side, and by the side of rivers, these oratories were used to be; Acts 16:13.
(m) T. Bab. Beracot, fol. 7. 1. Bereshit Rabba, sect. 56, fol. 50. 2. (n) De Vita Mosis, l. 3. p. 685. in Flaccum, p. 971, 972, 982. leg. ad Caium. p. 1011, 1012, 1013, 1014, 1016, 1040, 1043. (o) In Vita. (p) Abot R. Nathan, c. 4. fol. 2. 4. (q) T. Bab. Megilia, fol. 27. 2. (r) Satyr. 3. l. 295.

We often think one half hour a great deal to spend in meditation and secret prayer, but Christ was whole nights engaged in these duties. In serving God, our great care should be not to lose time, but to make the end of one good duty the beginning of another. The twelve apostles are here named; never were men so privileged, yet one of them had a devil, and proved a traitor. Those who have not faithful preaching near them, had better travel far than be without it. It is indeed worth while to go a great way to hear the word of Christ, and to go out of the way of other business for it. They came to be cured by him, and he healed them. There is a fulness of grace in Christ, and healing virtue in him, ready to go out from him, that is enough for all, enough for each. Men regard the diseases of the body as greater evils than those of their souls; but the Scripture teaches us differently.

THE TWELVE APOSTLES CHOSEN--GATHERING MULTITUDES--GLORIOUS HEALING. (Luke 6:12-49)
went out--probably from Capernaum.
all night in prayer . . . and when . . . day, he called, &c.--The work with which the next day began shows what had been the burden of this night's devotions. As He directed His disciples to pray for "laborers" just before sending themselves forth (see on Matthew 9:37; Matthew 10:1), so here we find the Lord Himself in prolonged communion with His Father in preparation for the solemn appointment of those men who were to give birth to His Church, and from whom the world in all time was to take a new mould. How instructive is this!

Went out into a mountain to pray. Preparatory to calling the apostles. Our Lord always prepared for any great crisis by prayer.

In the prayer of God - The phrase is singular and emphatical, to imply an extraordinary and sublime devotion. Mark 3:13.