Objection 1. It would seem that confession does not open Paradise. For different sacraments have different effects. But it is the effect of Baptism to open Paradise. Therefore it is not the effect of confession.
Objection 2. Further, it is impossible to enter by a closed door before it be opened. But a dying man can enter heaven before making his confession. Therefore confession does not open Paradise.
On the contrary, Confession makes a man submit to the keys of the Church. But Paradise is opened by those keys. Therefore it is opened by confession.
I answer that, Guilt and the debt of punishment prevent a man from entering into Paradise: and since confession removes these obstacles, as shown above (1,2), it is said to open Paradise.
Reply to Objection 1. Although Baptism and Penance are different sacraments, they act in virtue of Christ's one Passion, whereby a way was opened unto Paradise.
Reply to Objection 2. If the dying man was in mortal sin Paradise was closed to him before he conceived the desire to confess his sin, although afterwards it was opened by contrition implying a desire for confession, even before he actually confessed. Nevertheless the obstacle of the debt of punishment was not entirely removed before confession and satisfaction.
Source: Summa Theologica (New Advent)