Apex Trigger to Update Contact from Custom Object - triggers

Question

Long story short, I need to update a custom field in my standard Contact, that fires after a different, unrelated Custom Object is updated. I've tried to write a trigger that passes the field value from my Custom Object to the Contact, but I keep getting a variety of errors - the most recent of which has stumped me. The end goal is to update Passing__c from Passing_Field__c.
I'm getting an unexpected Token: "(" error on the for(Contact C: line. It is literally so simple I cannot figure it out.
Here is my code below. I've simplified the naming convention to try and make it more relatable. Any help is appreciated. I'm pretty new to Apex and Triggers, and I've been at this for a couple of hours now, hopefully some advice can send me to 'home plate'.
trigger ContactUpdater on Custom_Object_Name__c (after update) {
List<Contact> updatedContacts = new List<Contact>();
Set<Id> ObjectIds = new Set<Id>();
Set<String> ObjectCont = new Set<String>();
Set<Boolean> ObjectActive = new Set<Boolean>();
Set<String> ObjectPass = new Set<String>();
for(Custom_Object_Name__c p : trigger.new)
{
If(p.Active__c == true){
ObjectIds.add(p.Id);
ObjectCont.add(p.Contact__c);
ObjectActive.add(p.Active__c);
ObjectPass.add(p.Passing_Field__c);
}
try{ for(Contact c : [SELECT Id, Passing__c FROM Contact WHERE (AccountId IN (Select Account__c from Custom_Object_Name__c )) AND ObjectActive = true])
{
set(c.Passing__c = p.Passing_Field__c);
c.FieldToUpdate = c.Passing__c;
updatedContacts.add(c);
}
update updatedContacts;
}
catch(exception e){
throw e;
}
}
}
Notes: Active__c is a checkbox. Passing__c and Passing_Field__c are both text boxes.

Answer 1

I believe the issue is in the WHERE clause of your SOQL query:
WHERE (AccountId = (Select Account__c from Custom_Object_Name__c ))
Salesforce is expecting you to compare AccountId to an Id of some sort, rather than the results of some subquery. You probably want to try something like:
WHERE (AccountId in (Select Account__c from Custom_Object_Name__c))