How about "they just apparently believe it" because it is true? When a genuine believer is baptized in the Name of the Father, and of the Son, and of the Holy Ghost, it is understood by all that the true and living God is ONE GOD eternally existent as three divine Persons -- Father, Son, and Holy Spirit. Thus "name" is singular in Matthew 28:9 below (which is genuinely Scripture).When the trinity is taught, people do not have to infer it, nor imply it, nor even think about it, they just apparently believe it.
18 And Jesus came and spake unto them, saying, All power is given unto me in heaven and in earth. 19 Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost: 20 Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you alway, even unto the end of the world. (Mt 28:18-20)
For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one. (1 John 5:7 KJV)
16 And Jesus [the Son], when He was baptized, went up straightway out of the water: and, lo, the heavens were opened unto Him, and He saw the Spirit of God [the Holy Spirit] descending like a dove, and lighting upon Him: 17 And lo a [the Father's] voice from Heaven, saying, This is my beloved Son, in whom I am well pleased. (Mt 3:16,17)
Those who reject the doctrine of the Trinity reject the truth about God and are generally found among the cults.