1         The Status Quo 

If you look at the percentage of the world's population, Christianity has shrunk. In 1900, 36.6% of the world's population were followers of Christianity. In 2020, the share of Christians in the world population is estimated at 28.8%. Christianity has lost 7.8% of the world's population in 120 years. By comparison, Islam has grown from 13.1% to 25.6%, an increase of 12.5%. But where does this big difference come from, when Christianity has actually spread globally in the last 120 years? To do this, it is worth taking a mathematical look at how populations develop. 

TFR Christianity: 2,7 
TFR Islam: 3,1 

To keep up with Islam, Christianity would need about 8.5 million more people to join the group than to leave each year. Not in a revival phase for 10 years, but every single year in these 100 years. At 200 years, Christianity would need 18 million more people to join the group than to leave each year. Not for 10 years, not for 20 years, but for every single year in 200 years. Due to the compound interest effect, the need for an "immigration surplus" increases exponentially. It is obvious that the immigration surplus, no matter how successful missionary work is, comes to a limit where it is surpassed by the demographic advantage. 

The most likely scenario is that the TFR will decrease for both groups. However, the example shows the decisive factor that demographics or TFR differences have when it comes to the relative size of faith communities to the world population. If the TFR decreases but the TFR difference remains the same, the core message of the example above does not change. Suppose the TFR of both groups decreases by 1.0. Then Islam would be exactly at the replacement rate with a TFR of 2.1 and would remain nominally stable in the long term. Christianity, on the other hand, with a TFR of 1.7, would be below the sustaining rate and begin to shrink nominally within a generation. The TFR difference is the decisive factor why Christianity has shrunk by relatively 7.8% in 125 years. 

According to current forecasts, Christianity will no longer be the largest religious community in the world by 2070 at the latest. This is unsurprising, Christianity would need a TFR of at least 3,51, to keep pace with the demographically strongest community. 

2         Why the Status Quo Is Problematic for Christianity 

The Christian view of man is the foundation for human rights, which are considered universal today. Humanity is lost without the Christian image of man, which was brought into the world by professing Christians. This image of humanity will only exist in the future if the Christian community regains ground. 

Apostasy and conversions correlate with fertility rates 

Persecution of Christians and general hardship due to weak Christian demographics 

3         How we can solve the problem 

Most Important points:
Make Churches a place where young people can find a partner in a safe environment

Build up a culture/congregation in which young people can find each other and feel confident to marry

Build up a congregation with sense of family and support

Valuing children: Your wife will be like a fruitful vine within your house; your children will be like olive shoots  around your table. Behold, thus shall the man be blessed who fears the Lord. - Psalm 128: 3-4 English Standard Version

Other points:
Lifestyle can help: No alcohol, no obesity, practice NFP

Live a more modest lifestyle and recognize the real mission: You are the salt of the earth, but if salt has lost its taste, how shall its saltiness be restored? It is no longer good for anything except to be thrown out and trampled under people's feet. You are the light of the world. A city set on a hill cannot be hidden. Nor do people light a lamp and put it under a basket, but on a stand, and it gives light to all in the house. In the same way, let your light shine before others, so that they may see your good works and give glory to your Father who is in heaven. – Matthew 5: 13-16 English Standard Version
Be the salt and light of today and raise up the salt and light of tomorrow 

Develop a cultural sense of continuing killed brothers and sisters with own children 

Make the clergy aware of the topic and make them to advocates 

Demographic education: Make all Christians aware of the importance of the TFR; Make all Christians aware that the fertility declines significant with age 35 (especially woman but also man) 

Build up communities which contribute to higher TFR: Groups like Orthodox Judaism or TLM-Catholicism could serve as inspiration 

Do not be held back by false prospects; indeed, the higher the Christian TFR, the higher the percentage in the global population, the better the outlook 

Have a vision:  It may be that Jesus will come back in an hour or tomorrow or it may take another 2000 years or longer. Suppose it takes another 2000 years or more. What should Christianity look like? Should it be a marginalized small group that experiences apocalyptic suffering, without any “end times”, because we do not have the size to reduce or completely prevent this suffering, as we do today in Iraq? Or should Christianity be there when humanity populates the entire universe? One vision is that (if Christ does not come sooner) the whole galaxy will be populated by trillions of Christians and the whole universe will vibrate with praise. Maybe then it will be the day when no one expects it and Jesus will come back. Imagine that your descendants live on the planets and glorify God, who orbit around the stars that Abraham looked up to thousands of years before us.