There is one body and one Spirit—…one hope…—one Lord, one faith, one baptism; one God and Father of all, who is over all and through all and in all. Eph 4:5-6 NLT
All true believers are a part of the body of Christ, have the same Spirit, and the same faith. Denominations and sects do not represent “different forms of Christianity” any more than people of different colors and languages represent different races of humans. There is one human race with one blood exhibiting many genetic variables. There is one body of Christ (form of Christianity) manifesting through many doctrinal facets. The Cross: salvation through the blood of Jesus Christ by faith is the central doctrine of all true Christians.
Paul also made it clear there is no other gospel.
Paul, an apostle—sent not from men nor by man, but by Jesus Christ and God the Father, who raised him from the dead—… To the churches in Galatia: Grace and peace to you from God our Father and the Lord Jesus Christ, who gave himself for our sins to rescue us from the present evil age, according to the will of our God and Father, to whom be glory for ever and ever. Amen.
I am astonished that you are so quickly deserting the one who called you by the grace of Christ and are turning to a different gospel— which is really no gospel at all. Evidently some people are throwing you into confusion and are trying to pervert the gospel of Christ. But even if we or an angel from heaven should preach a gospel other than the one we preached to you, let him be eternally condemned! As we have already said, so now I say again: If anybody is preaching to you a gospel other than what you accepted, let him be eternally condemned! Gal 1:1-9 NLT
Succinctly, there is only one way to respond to the gospel and that is to accept it by faith. Jesus died for our sins and rose again to give us eternal life. Jesus said, “…unless you believe that I Am who I claim to be, you will die in your sins.” How much clearer could it be?