Objection 1. It would seem that John's baptism should have ceased after Christ was baptized. For it is written (John 1:31): "That He may be made manifest in Israel, therefore am I come baptizing in water." But when Christ had been baptized, He was made sufficiently manifest, both by the testimony of John and by the dove coming down upon Him, and again by the voice of the Father bearing witness to Him. Therefore it seems that John's baptism should not have endured thereafter.
Objection 2. Further, Augustine says (Super Joan., Tract. iv): "Christ was baptized, and John's baptism ceased to avail." Therefore it seems that, after Christ's baptism, John should not have continued to baptize.
Objection 3. Further, John's baptism prepared the way for Christ's. But Christ's baptism began as soon as He had been baptized; because "by the touch of His most pure flesh He endowed the waters with a regenerating virtue," as Bede asserts (Mag. Sent. iv, 3). Therefore it seems that John's baptism ceased when Christ had been baptized.
On the contrary, It is written (John 3:22-23): "Jesus... came into the land of Judea... and baptized: and John also was baptizing." But Christ did not baptize before being baptized. Therefore it seems that John continued to baptize after Christ had been baptized.
I answer that, It was not fitting for the baptism of John to cease when Christ had been baptized. First, because, as Chrysostom says (Hom. xxix in Joan.), "if John had ceased to baptize" when Christ had been baptized, "men would think that he was moved by jealousy or anger." Secondly, if he had ceased to baptize when Christ baptized, "he would have given His disciples a motive for yet greater envy." Thirdly, because, by continuing to baptize, "he sent his hearers to Christ" (Hom. xxix in Joan.). Fourthly, because, as Bede [Scot. Erig. Comment. in Joan.] says, "there still remained a shadow of the Old Law: nor should the forerunner withdraw until the truth be made manifest."
Reply to Objection 1. When Christ was baptized, He was not as yet fully manifested: consequently there was still need for John to continue baptizing.
Reply to Objection 2. The baptism of John ceased after Christ had been baptized, not immediately, but when the former was cast into prison. Thus Chrysostom says (Hom. xxix in Joan.): "I consider that John's death was allowed to take place, and that Christ's preaching began in a great measure after John had died, so that the undivided allegiance of the multitude was transferred to Christ, and there was no further motive for the divergence of opinions concerning both of them."
Reply to Objection 3. John's baptism prepared the way not only for Christ to be baptized, but also for others to approach to Christ's baptism: and this did not take place as soon as Christ was baptized.
Source: Summa Theologica (New Advent)