What things should I be open about with my spouse, and what things can I keep private?
Whenever we marry someone they bring to the relationship a past. Going into a marriage each spouse should have a good understanding about their mate's personal history, their likes, dislikes, habits, activities, values, beliefs, and plans for the future.
These are important things to share with each other before we make the big decision.
What is not something we need to share with our mate are past sins that have been forgiven and are not being repeated in that person's life anymore, and would not impact the marriage in any way.
Example: When Joe was 15 years old he was caught shop-lifting and was arrested. He is now 30 and has never shop-lifted since. He repented of this and was forgiven. Although he is free to tell his bride-to-be about this, it would not be wrong if he kept it between him and the Lord.
But ...
What if Joe had a chronic habit of shop-lifting. He keeps thinking the last incident would be his last--yet he continues to struggle with stealing. Does Joe need to be open and honest about this? Absolutely.
The same would hold true of past mistakes that could affect a spouses trust for that person. Past infidelities, porn use, alcoholism, past gambling problems, etc. These types of things should all be out in the open.
As far as current life together, I respect the saying, "A person who has nothing to hide--hides nothing." It is reasonable that our spouse should know where we are, who we are with, what activity we are doing, and what our thoughts and feelings are about in all areas, etc.
Some spouses would like to call secrecy- "privacy"...but in an open and honest marriage we should be able to have private time (meaning alone time without our spouse) but there should still be no 'secret' as to what we are doing, where we are, who we are with, etc.
Leave secrets for birthdays and Christmas. Be open and honest with your spouse about everything else.