I am a baptized Christian and have been attending RCIA. I was just told that to be accepted into the Church, I will need an annulment from a previous marriage. What is the reasoning for this?
Presumably, you have remarried after your divorce. The reasoning is that you are currently living in what must be presumed to be an invalid marriage and, in that invalid marriage, sharing marital relations with your spouse. The Church sees that as an ongoing case of adultery. As adultery is grave matter, the deliberate choice to engage in adultery can be a mortal sin. Mortal sin bars a person from Communion. Ordinarily, this means that you must receive an annulment for your first marriage and have your second marriage regularized by the Church before you can be received into the Church.
If you would like to be received into the Church pending the annulment, there is a second option. If you and your spouse agree to live as brother and sister until an annulment can be obtained – in other words, the two of you agree to abstain from marital relations until your marriage can be regularized – you can be received into the Church now.