So I looked through some old threads and didn't see a lot about this. For that reason, I thought I'd get other players input. How can you be the best celestial templar at low to mid levels (1st - 10th)? What has worked for you?
So, my classic concept of a templar would have you starting with some sort of one-handed stick, a style (probably shield, but two weapons can be interesting), and maybe a level of Blacksmith (if you want to self-adjust your armor). Then, R/W, Read Magic, and a 3/2 tree. That will take you 30 build (assuming OHB/Shield and you're not paying double for R/W).
Your strongest time at this juncture is going to be daylight - using Flame Bolt/Stone Storm/Lightning Storm scrolls in the same way you would use a slay: as punctuation on the damage wall, Disarm/Ping/Repel/Bind/Shatter/Web for selective tactical combat, etc. You're likely going to want to memorize Disarms and Magic Armors. Shield scrolls, at this level, are also a good choice because they represent a reasonable percentage of the total damage you can take. For potions: lots of defensives: Magic Armor and Spell Shield, plus a couple of Cure Serious in case you land in a tough spot away from your healers.
Mastering the "templar tuck" is going to be key here - most of your casting is going to be close up, so you need to be able to tuck your weapon, throw from your waist, and bring the weapon back up quickly. If you're playing a team-based character, now is also a good time to start practicing sticking just behind the shoulder of your party's front-line fighter. Your job here isn't necessarily to fight shoulder-to-shoulder, but primarily to cover your teammates flank defensively - give her room to do the big damage swings, as well as being ready to put a new Magic Armor or Spell Shield on her.
Next up, you want to run up your fighter skills. Prof 1, prof 2, then a parry take you to 71 build. If you find yourself in more night fights than daylight/well-lit encounters, you might want to pick up slay and a couple of Disarms, but imo, you're better off using scrolls for equivalents and leave the real slays to the fighters. The overall rule of thumb is that if it's already covered under your spells and scrolls, don't worry about them too much. IMO, your goal as you level up beyond this point, ideally, is to use your fighter skills to cover things you can't do with your spells. Two more profs and another parry bring you to 112 build, on the top edge of 10th level. Again, if you're playing for a team, your role is still to cover the flank, but you're also popping a bit more damage so you're a bit more active.
Beyond 10th level, I'd say fill in a couple other fighter skills you don't have spell equivalents for first: Eviscerate, Riposte, Stung Limb, in that order. You're swinging for 6 naturally, 9 with a +3 weapon. And more profs at at this stage has you waiting three or four events for each one (ymmv depending on gobbieing). A decent stagger in build spending here is 20 build in magic, then 20 in fighter skills, then 20 in magic again, etc.