It is always OK to have your own beliefs. Always. No exceptions.
It's OK to follow your nose. There is no coordinating, over-seeing ritual police in witchcraft & no governing body that dictates rules. That is a hard thing to get used to after even scant exposure to a mainstream religion. Ritual format is a procedure that is logical to the function & can be changed to fit you.
Substituting and deity plug-&-play is a bit tricky. You work with the energy concepts they represent. Even though I'm Christian & quarters can be assigned to the 4 arch-angelic forces I simply relate better to earth elementals for quarters because the Earth is the Temple. I would not feel comfortable calling Yahweh or Jesus to witness my circle. The 1st would blow steam out his ears & the other would have a giggle fit; &'the one time I'd tried it I dissolved on the floor into knee-slapping laughter. It could have been my bias coming thru, but I knew I had to deal with it. For one thing not every circle needs the rites of quarter calling & invitation to deity. Try to keep it simple. The rituals are not a solemn dirge in a pew-format church. You can get up & move, make noise, laugh. You play all the parts: choir, cantor & pastor.
Define to yourself the natural forces your deity concept represents as you understand it. Since you haven't read the stories you may get along with just an overall birth, death, resurrection theme found in nature & the Green Man. JC referred to himself as the stalk of wheat that must first fall for it's seed to regrow &'resurrect. This is a reference by the man himself (or the author) to the Sumerian god Tammuz. So if he could make the agrarian cult connection, why not use it?
I took a clue from ceremonial magical practice that works with aspects as energies & not as beings, as this was what Gerald Gardner apparently had in mind when he consulted Crowley for Wiccan rituals. It's Nature that holds the symbolism.
Mostly I don't call on a god-form in ritual by name but by titles of Primordial Mother/Star Goddess & her son, the Horned God of Nature & then let the ethers sort it out. it's Nature as the primary jewel of all god-form's creation that I want to evoke, & not a specific god-form. But that's only me. You have to work with your own beliefs.
When you ask "what about altar work"? ... Well, for goodness sake, what
about it? There are no altar police either
. Keep a devotional altar if you want one & are comfortable with it. It doesn't need to hold anything that doesn't symbolize or connect to your concept of deity. Sooner or later you need to know to who or what you address those prayers. It's not the place to drop your car keys. Write down some ideas of what you might use as symbols. A cup of water is an acceptable offering but you don't need it if that's not your style. Play around with it to find out what your style IS.
If you want a christian theme there is no shortage of decor. Look at "personal altars" images on your browser as opposed to "wiccan altars" images here on the forum. Maybe keep a Psalms & New Testament Bible handy for reading. Psalms are devotional prayers & many can be adapted as spells. there are oodles of books on devotional practices.
As I noted in "getting started" a working altar for ritual & magic is different from a devotional altar, but lots of people combine them. It's your choice.