"Then Philip went down to the city of Samaria and preached Christ to them. And the multitudes with one accord heeded the things spoken by Philip, hearing and seeing the miracles which he did. For unclean spirits, crying with a loud voice, came out of many who were possessed; and many who were paralyzed and lame were healed. And there was great joy in that city. But there was a certain man called Simon, who previously practiced sorcery in the city and astonished the people of Samaria, claiming that he was someone great, to whom they all gave heed, from the least to the greatest, saying, 'This man is the great power of God.' And they heeded him because he had astonished them with his sorceries for a long time. But when they believed Philip as he preached the things concerning the kingdom of God and the name of Jesus Christ, both men and women were baptized. Then Simon himself also believed and when he was baptized he continued with Philip, and was amazed, seeing the miracles and signs which were done" (Acts 8:5-13).
Among the many who were scattered everywhere preaching the word, Luke chooses to relate the labors here of only one. Philip, who was one of the seven (cf. 6:5; not the apostle - cf. 8:1), descended ("went down") from the elevated city of Jerusalem as the persecution intensified and traveled north to Samaria. Jesus had spent some time there during His ministry, despite the fact that Jews and Samaritans typically hated each other, and had prepared the soil for a great spiritual harvest (e.g., John 4). Jesus had predicted the gospel would first be preached in Jerusalem and then into Judea and Samaria (cf. Acts 1:8)--and so it was!
Philip "preached Christ" to them. As we will see more clearly as this chapter progresses, to preach Christ is to explain all about Christ, His church, and the way of salvation (which includes the fundamentals of the scheme of redemption, facts about Jesus, as well as the necessary response of faith, repentance, immersion, and perseverance).
The multitudes responded favorably to Philip's preaching. They were glad to obey his words, being persuaded of the divine origin of his message by the numerous miracles Philip worked. For example, he cast out evil spirits and healed many who were paralyzed and lame. Great joy resulted in these good and honest hearts from these healings and from salvation being freely offered to all. Remember, Philip had received this miraculous power when the apostles had laid their hands on him (cf. Acts 6:6-8).
Before Philip came there was another man, Simon, who amazed the people with his "great power" "claiming that he was someone great." In truth, Simon was nothing more than a professional magician who was involved in the occult. He had no real power, per se, but he was a master of illusion and deception. He was likely heavily involved in forbidden practices of the occult since the people believed his power was from God (though interestingly, there is no record of him healing people as Philip did). He was pleased with that reputation and did not dispel the myth (though he knew better). Simon was well known in the area for he had astonished people with his magic arts for a long time.
How would Simon view Philip? Would he perceive him to be a threat to his popularity or livelihood? Apparently, Simon is so impressed with Philip that he has no ill will toward him. Both Simon and many men and women from Samaria believed what was preached concerning the kingdom of God and Jesus and were baptized to have their sins forgiven. We see here, as is also the case elsewhere in the book of Acts, that it is impossible to genuinely believe the gospel without obeying it (which includes submitting to water baptism). Also, it is clear that one should not attempt to separate the preaching of Christ (cf. 8:5) from the preaching of His church or kingdom (cf. 8:12).
Simon, after being baptized, tagged along with Philip being amazed at the miracles and signs being done. The validity of the miracles is underscored by Simon's conversion. No doubt Simon, experienced as he was, knew a good trick when he saw one (or performed one), but the acts of Philip were real. Unlike himself, Simon knew there was no deception with Philip, only genuine power from God! He was fascinated and wanted to learn more. There is no doubt that his conversion was sincere (since the historian says "Simon also believed"), though his lust for power will soon lead him to make a serious mistake. If one questions the conversion of Simon, based upon what is learned later, he must also question (in order to be consistent) the conversion of all the Samaritans since Simon responded just as they did!