Was Christianity made up for political gain?

No, it was not so. It seems you missed the history of early Christians (A Look at the Early Church) which helps you understand how it developed.

Christianity developed out of Judaism in the 1st century C.E. It is founded on the life, teachings, death, and resurrection of Jesus Christ, and those who follow him are called "Christians." Until the latter part of the 20th century, most adherents of Christianity were in the West, though it has spread to every continent and is now the largest religion in the world. Traditional Christian beliefs include the belief in the one and only true God, who is one being and exists as Father, Son, and Holy Spirit, and the belief that Jesus is the divine and human Messiah sent to the save the world.

Roman Empire

Paul established Christian churches throughout the Roman Empire, including Europe, and beyond - even into Africa.

Persecution

However, in all cases, the church remained small and was persecuted, particularly under tyrannical Roman emperors like Nero (54-68), Domitian (81-96), under whom being a Christian was an illegal act, and Diocletian (284-305).

Many Christian believers died for their faith and became martyrs for the church (Bishop Polycarp and St Alban amongst others).

Constantine turns the tide

When a Roman soldier, Constantine, won victory over his rival in battle to become the Roman emperor, he attributed his success to the Christian God and immediately proclaimed his conversion to Christianity.

Christianity thus became the official religion of the Roman Empire.

Despite its humble origins, the Church has made more changes on earth for good than any other movement of force in history. To get an overview of some of the positive contributions Christianity has made through the centuries, here are a few highlights:

(1) Hospitals, which essentially began during the Middle Ages.

(2) Universities, which also began during the Middle Ages. In addition, most of the world’s greatest universities were started for Christian purposes.

(3) Literacy and education for the masses.

(4) Capitalism and free enterprise.

(5) Representative government, particularly as it has been seen in the American experiment.

(6) The separation of political powers.

(7) Civil liberties.

(8) The abolition of slavery, both in antiquity and in more modern times.

(9) Modern science.

(10) The discovery of the New World by Columbus.

(11) The elevation of women.

(12) Benevolence and charity; the good Samaritan ethic.

(13) Higher standards of justice.

(14) The elevation of common man.

(15) The condemnation of adultery, homosexuality, and other sexual perversions. This has helped to preserve the human race, and it has spared many from heartache.

(16) High regard for human life.

(17) The civilizing of many barbarian and primitive cultures.

(18) The codifying and setting to writing of many of the world’s languages.

(19) Greater development of art and music. The inspiration for the greatest works of art.

(20) The countless changed lives transformed from liabilities into assets to society because of the gospel.

(21) The eternal salvation of countless souls.

The last one mentioned, the salvation of souls, is the primary goal of the spread of Christianity. All the other benefits listed are basically just by-products of what Christianity has often brought when applied to daily living.