Use your Wizard (Theurge) level
The Channel Arcana feature at second level uses your wizard level.
At 2nd level, you gain the ability to channel
arcane energy directly from your deity, using that energy to fuel magical effects. You start with two such effects: Divine Arcana
and the Channel Divinity option granted at 2nd level by your chosen domain. You employ that Channel Divinity option by using your Channel Arcana ability.
The channel divinity option that you chose, Preserve Life, is one of two ways for you to use your Channel Arcana option. All Channel Arcana options are tied to your wizard level, and by selecting that option it becomes a wizard ability, not a cleric ability1.
(The need to use your holy symbol appears to remain, as this UA/Playtest article does not waive that requirement, however, it would be a fair ruling to sub in the arcane focus for holy symbol there ... work that out with your DM).
1 If Theurge required multiclassing to cleric, it would say so. It does not require that, thus the borrowed channel divinity becomes Channel Arcana and is based on the PC's level in the Theurge class: Wizard.