Theophylact of Ohrid

"Simon himself also believed." He believed not for the sake of faith, but so that he too might work miracles. How did he think to work miracles? He himself deceived, and also subdued the demon-possessed, and therefore thought that the apostles likewise made use of some art. That is why he also offered money. Again for the same reason, so as not to be deprived of this gift, he constantly stayed near Philip.

Commentary on Acts ↗

Oecumenius

Thus Simon, although he was a magician and flattered the apostles, and although, like others, he had been baptized by Philip, who was one of the seven, he nevertheless did not receive the Holy Spirit through the laying on of the apostles' hands. What then? Did those baptized by Philip not receive the Holy Spirit? They received the Holy Spirit of remission, but they did not receive the Spirit of miracles. Therefore Simon, seeing that it is given through the laying on of hands, came to the apostles, daring to trade with him. …

Commentary on Acts ↗

Bede

Then Simon himself also believed. Either he truly believed in the Lord, having been overcome by the words and virtue of the blessed Philip, or, as it is more credible, he pretended to believe until he received baptism, so that, because he was very eager for praise, in such a way that he wanted to be considered the Christ, as the stories tell, he might learn from Him the arts by which he performed miracles. …

Commentary on Acts ↗