Now if Christ is proclaimed as raised from the dead, how can some of you say there is no resurrection of the dead? If there is no resurrection of the dead, then Christ has not been raised; and if Christ has not been raised, then our proclamation has been in vain and your faith has been in vain. We are even found to be misrepresenting God, because we testified of God that he raised Christ—whom he did not raise if it is true that the dead are not raised. For if the dead are not raised, then Christ has not been raised. If Christ has not been raised, your faith is futile and you are still in your sins. Then those also who have died in Christ have perished. If for this life only we have hoped in Christ, we are of all people most to be pitied.
(Lectionary, New Revised Standard Version)
We should remember that some of the epistles (letters) in New Testament were written before the Gospels, and Paul's letter to the Christians in Corinth would appear to be dated to around 53-57 CE (around 20 years after the death of Jesus). In Chapter 15 he addresses a number of issues about the question of whether Jesus rose from the dead, and he is scathing of those who were arguing against the resurrection.
Some of those who were Christians in Corinth might have felt that Jesus rose from the dead, but for others ideas of Greek philosophy that spirit was superior to flesh meant that they couldn't believe that there was life after death. Some Jews were also perhaps influenced by the Sadducees who thought that anything beyond this life was wishful thinking. Paul points out that if the Corinthians don't believe in the resurrection then evil and death defeated Jesus, and if so He is not God. If Jesus did not rise from the grave then He did not offer a sacrifice for our sin, and He cannot save us.
Many Christians have struggled with the thought of the Resurrection, but we should remember that millions of Christians have said for centuries these words in the Nicene Creed:
"We believe in one Lord Jesus Christ....
He suffered death and was buried.
On the third day He rose again,"
If our faith grows dim at any point, others will hold the belief for us until we can say the words with confidence once again.
Lord Jesus Christ,
when we struggle to believe
in the words we say in the Creeds
give us faith to understand
that You came to earth as a man
to rescue us from the power of darkness.
You came to re-unite us with God once more
and offered Your life as restitution for our sin.
May we learn to accept this with simple faith,
for with God all things are possible.
Amen.
You might like to look at this article on the two Creeds that are commonly used in Church today: