What is the northernmost region on the map into which Christianity had spread by A. D. 325?

Asia Minor.

Explanation:

Christianity began spreading in the 1st century AD after the crucifixion and resurrection of Jesus Christ. The apostles of Jesus Christ began preaching the message of the Kingdom of God and the gospel of Christ after witnessing the resurrected Jesus.

Apostle Paul was one of the apostles who played a major role in spreading the gospel to many parts of the world. He was not one of the twelve disciples of Jesus but was chosen later.

In the 4th century, Christianity began spreading rapidly. The region in which Christianity spread by A D 325 was Asia Minor. Asia Minor was the region that was referred to Roman Province. Constantine 1 was the first Roman Empire to convert in Christianity. In 325 AD, he called for the First Council of Nicea, which brought the Nicene Creed.

So, the correct answer is Asia Minor.