Objection 1. It seems that it is not the effect of Baptism, to open the gates of the heavenly kingdom. For what is already opened needs no opening. But the gates of the heavenly kingdom were opened by Christ's Passion: hence it is written (Apocalypse 4:1): "After these things I looked and behold (a great) door was opened in heaven." Therefore it is not the effect of Baptism, to open the gates of the heavenly kingdom.
Objection 2. Further, Baptism has had its effects ever since it was instituted. But some were baptized with Christ's Baptism, before His Passion, according to John 3:22-26: and if they had died then, the gates of the heavenly kingdom would not have been opened to them, since none entered therein before Christ, according to Micah 2:13: "He went up [Vulgate: 'shall go up'] that shall open the way before them." Therefore it is not the effect of Baptism, to open the gates of the heavenly kingdom.
Objection 3. Further, the baptized are still subject to death and the other penalties of the present life, as stated above (Article 3). But entrance to the heavenly kingdom is opened to none that are subject to punishment: as is clear in regard to those who are in purgatory. Therefore it is not the effect of Baptism, to open the gates of the heavenly kingdom.
On the contrary, on Luke 3:21, "Heaven was opened," the gloss of Bede says: "We see here the power of Baptism; from which when a man comes forth, the gates of the heavenly kingdom are opened unto him."
I answer that, To open the gates of the heavenly kingdom is to remove the obstacle that prevents one from entering therein. Now this obstacle is guilt and the debt of punishment. But it has been shown above (1,2) that all guilt and also all debt of punishment are taken away by Baptism. It follows, therefore, that the effect of Baptism is to open the gates of the heavenly kingdom.
Reply to Objection 1. Baptism opens the gates of the heavenly kingdom to the baptized in so far as it incorporates them in the Passion of Christ, by applying its power to man.
Reply to Objection 2. When Christ's Passion was not as yet consummated actually but only in the faith of believers, Baptism proportionately caused the gates to be opened, not in fact but in hope. For the baptized who died then looked forward, with a sure hope, to enter the heavenly kingdom.
Reply to Objection 3. The baptized are subject to death and the penalties of the present life, not by reason of a personal debt of punishment but by reason of the state of their nature. And therefore this is no bar to their entrance to the heavenly kingdom, when death severs the soul from the body; since they have paid, as it were, the debt of nature.
Source: Summa Theologica (New Advent)