Not only is Namu Myoho renge kyo the seed (cause) of Buddhahood but the effects of Buddhahood

"If it is not the proper Object of Worship (Gohonzon), even though there be no falsehood [even if the practitioner is sincere] it will not form the seed [of Buddhahood]. The Daimoku of the Hommon [Doctrine of the Original, found in the Lotus Sutra] is the seed of Buddhahood. This is because the Eternal Shakyamuni of the Honmon has put his Causal Practices (his myriad practices that lead to enlightenment) and his Virtues of the Effect (the merits of the effect, which is Buddhahood) into the seven characters of Namu Myo Ho Renge Kyo." - The True Object of Worship

Those who practice Namu Myoho renge kyo with a proper Object of Worship (or devotion) manifest the virtues and merits of the Buddha. Those who take Jesus, Mohammad, Nichiren, or SGI as their Object of Worship, never manifests such virtue and merits.