Shirk is the one unforgivable sin in Islam, if one dies in this state.:
Associating a partner or others with Allah is a rejection of Islam and takes one outside of the faith. The Quran says:
"Verily, Allah forgives not the sin of setting up partners in worship with Him, but He forgives whom He wills sins other than that. And whoever sets up partners in worship with Allah, has indeed strayed far from the path." (4:116)
-Thus to call Jesus God is unforgivable.
For Christians the opposite is true. If you don't believe Jesus is God you go to hell.
Romans 10:9 because if you acknowledge
and confess with your mouth that Jesus is Lord [recognizing His power, authority, and majesty as God], and believe in your heart that God raised Him from the dead, you will be saved. 10 For with the heart a person believes [in Christ as Savior] resulting in his justification [that is, being made righteous—being freed of the guilt of sin and made acceptable to God]; and with the mouth he acknowledges
and confesses [his faith openly], resulting in
and confirming [his] salvation.
Conclusion: You cannot be a 'good' Christian (or even a Christian) and be a 'good' Muslim. Similarly, you cannot be a 'good' Muslim (or even a Muslim) and be a 'good' Christian