Objection 1. It would seem that Christ did not open the gate of heaven to us by His Passion. For it is written (Proverbs 11:18): "To him that soweth justice, there is a faithful reward." But the reward of justice is the entering into the kingdom of heaven. It seems, therefore, that the holy Fathers who wrought works of justice, obtained by faith the entering into the heavenly kingdom even without Christ's Passion. Consequently Christ's Passion is not the cause of the opening of the gate of the kingdom of heaven.
Objection 2. Further, Elias was caught up to heaven previous to Christ's Passion (2 Kings 2). But the effect never precedes the cause. Therefore it seems that the opening of heaven's gate is not the result of Christ's Passion.
Objection 3. Further, as it is written (Matthew 3:16), when Christ was baptized the heavens were opened to Him. But His baptism preceded the Passion. Consequently the opening of heaven is not the result of Christ's Passion.
Objection 4. Further, it is written (Micah 2:13): "For He shall go up that shall open the way before them." But to open the way to heaven seems to be nothing else than to throw open its gate. Therefore it seems that the gate of heaven was opened to us, not by Christ's Passion, but by His Ascension.
On the contrary, is the saying of the Apostle (Hebrews 10:19): "We have [Vulgate: 'having a'] confidence in the entering into the Holies"--that is, of the heavenly places--"through the blood of Christ."
I answer that, The shutting of the gate is the obstacle which hinders men from entering in. But it is on account of sin that men were prevented from entering into the heavenly kingdom, since, according to Isaiah 35:8: "It shall be called the holy way, and the unclean shall not pass over it." Now there is a twofold sin which prevents men from entering into the kingdom of heaven. The first is common to the whole race, for it is our first parents' sin, and by that sin heaven's entrance is closed to man. Hence we read in Genesis 3:24 that after our first parents' sin God "placed... cherubim and a flaming sword, turning every way, to keep the way of the tree of life." The other is the personal sin of each one of us, committed by our personal act.
Now by Christ's Passion we have been delivered not only from the common sin of the whole human race, both as to its guilt and as to the debt of punishment, for which He paid the penalty on our behalf; but, furthermore, from the personal sins of individuals, who share in His Passion by faith and charity and the sacraments of faith. Consequently, then the gate of heaven's kingdom is thrown open to us through Christ's Passion. This is precisely what the Apostle says (Hebrews 9:11-12): "Christ being come a high-priest of the good things to come... by His own blood entered once into the Holies, having obtained eternal redemption." And this is foreshadowed (Numbers 35:25-28), where it is said that the slayer* "shall abide there"--that is to say, in the city of refuge--"until the death of the high-priest, that is anointed with the holy oil: but after he is dead, then shall he return home." [The Septuagint has 'slayer', the Vulgate, 'innocent'--i.e. the man who has slain 'without hatred and enmity'.]
Reply to Objection 1. The holy Fathers, by doing works of justice, merited to enter into the heavenly kingdom, through faith in Christ's Passion, according to Hebrews 11:33: The saints "by faith conquered kingdoms, wrought justice," and each of them was thereby cleansed from sin, so far as the cleansing of the individual is concerned. Nevertheless the faith and righteousness of no one of them sufficed for removing the barrier arising from the guilt of the whole human race: but this was removed at the cost of Christ's blood. Consequently, before Christ's Passion no one could enter the kingdom of heaven by obtaining everlasting beatitude, which consists in the full enjoyment of God.
Reply to Objection 2. Elias was taken up into the atmospheric heaven, but not in to the empyrean heaven, which is the abode of the saints: and likewise Enoch was translated into the earthly paradise, where he is believed to live with Elias until the coming of Antichrist.
Reply to Objection 3. As was stated above (Question 39, Article 5), the heavens were opened at Christ's baptism, not for Christ's sake, to whom heaven was ever open, but in order to signify that heaven is opened to the baptized, through Christ's baptism, which has its efficacy from His Passion.
Reply to Objection 4. Christ by His Passion merited for us the opening of the kingdom of heaven, and removed the obstacle; but by His ascension He, as it were, brought us to the possession of the heavenly kingdom. And consequently it is said that by ascending He "opened the way before them."
Source: Summa Theologica (New Advent)