Martel Temple is located just north of Iselia's north exit.
When you enter, you will meet a dying pastor who explains the situation at the temple. After the conversation, you will be given a choice of what Lloyd is to say. One of many throughout the game that will influence relationship levels with other characters. Climb the stairs to initiate a cutscene followed by a boss battle.
Boss: Vidarr Be sure to use the Magic Lens you received earlier on him. Vidarr wields a large ball and chain as well as a large hammer. He is quite slow, but packs quite a bit of power. Have Genis stay back and cast Fireball while Colette and Lloyd rush him. The safest tactic is to complete one combo, and then guard against his attack. Once Vidarr is brought down to about half HP, the battle will be interrupted by a cutscene, during which Kratos will join your team. The additional ally greatly helps, not to mention that he can cast attack magic such as Fireball as well as the healing spell, First Aid. Once Kratos joins, the battle will go fairly quick.
After a cutscene, you can enter the temple. Your path forward is blocked by a magical barrier so run down the stairs in one of the side halls to reach the basement. In the basement you will find a glass floor suspended over a few narrow paths. The paths all have chunks missing and can't be traversed. There is also a single Golem wandering around. Approach it to fight and defeat it. It will turn into a block that you can push around. Drop the block down onto the pathways. To reach everything, you will need to continuously fight golems and drop their blocks through the floor. Eventually, you will be able to approach the altar at the far north of the temple. From there you will gain the Sorcerer's Ring. Using it fires a small fireball forward that can stun enemies. Exit the basement and return to the magic barrier. Use the ring's fire to unlock it and continue into the teleporter. After the cutscene, return to Iselia.