The part where you say, "it seems like they really are two totally different religions" is where the crux of the matter is.
Christianity is built upon Judaism, and is an extension of the promises of those of the faith, Abraham, Isaac, Jacob (Israel) and usually I do not use this name, but King David, since Jesus was referred to as "Son of David."
Faith in Jesus Christ cannot be seen as a religion.
Similar to the faith of Abraham.
The manifestations of God the Son, Jesus, when he appeared to Abraham, on the way down to Sodom, and in his reassurance to Abraham of the Son of Promise.
The faith of Abraham, which was imputed to him by God for righteousness, Abraham being called God's Friend, which is a very special distinction, similar to David, being called "a man after God's own heart."
There is so much evidence of the two tied faiths of Abraham and David and that in Jesus Christ, placed by Gentiles, and Jews who are Messianic, that all one has to do is look for the similarities, and allow God to bring revelation through the Holy Spirit, who did raise Jesus Christ from the dead, after his Crucifixion on the Cross of Calvary as the Finished Work, that was started from the Garden of Eden, blood sacrifice, without which there is no remission of sin.
Only in the case of Jesus Christ, the continual sacrifice was curtailed, "It is Finished", and Jesus Christ would not die for the sins of the world a second time.
As far as the Trinity goes, the laws of "in the mouth of two or two witnesses shall a thing be established," is not only applied for use on earth.
John 18V37:"Pilate therefore said unto him, Art thou a king then? Jesus answered, Thou sayest that I am a king. To this end was I born, and for this cause came I into the world, that I should bear WITNESS unto the truth. Every one that IS of the truth heareth my voice."
ISAIAH 55V3ff:"Incline your ear, and come unto me: hear , and your soul shall live, and I will make an Everlasting Covenant with you, even the sure mercies of David.
V4:"Behold, I have given him for a WITNESS to the people, a leader and commander to the people.
V5:"Behold, thou shalt call a nation that thou Knowest Not, and nations that Knew Not thee shall run unto thee because of the Lord thy God, and for the Holy One of Israel; for he hat glorified thee.
v6:"Seek ye the Lord while he may be found, Call ye upon him while he is near:"
The eyes of the Jews are blinded, (no faith in Jesus Christ as God the Son and Redeemer/Saviour) without which, the Gentiles prophecy of Isaiah could not be fulfilled.
1John 5V6:"This is he that came by water and blood, even Jesus Christ; not by water only, but by water and blood.
And it is the Spirit that beareth WITNESS, because the Spirit is truth.
V7:"For there are three that bear RECORD in heaven, the Father, the Word, and the Holy Ghost (Spirit); and these three are one.
V8:"And there are three that bear WITNESS in earth, the spirit, and the water, and the blood, and these three Agree in one.
V9:"If we receive the WITNESS of men, the WITNESS of God is GREATER: for this is the WITNESS of God which he hat testified of his Son.
V10:"He that believeth on the Son of God hath the WITNESS in himself; he that believeth not God hat made him a liar; because he believeth NOT the RECORD that God gave of his Son.
V11:"And this is the RECORD, that God hath given to us eternal life, and this life is in his Son.
V12:"He that hath the Son hath life; and he that hath NOT the Son of God hath not life."
Having given just a few biblical quotes to support this, I would say to anyone, especially to any Jewish person, Abraham did not have the Torah and all the OT was written by Holy men of God, by inspiration of the Holy Spirit of God.
Since a false prophet could not stand up against God's Holy Spirit, if one is truly a believer, in the God of Abraham, Isaac and Jacob, it would make no sense, that people who put faith in Jesus Christ/the Triune Godhead, why has not anyone been struck down, for blaspheming God of Abraham, Jacob, Isaac, name and Holy Word.
Christians are fully aware of the Sovereignty of God and His Holiness, Righteousness and Power.
If we put faith in Jesus Christ, and the Holy Spirit, and this was against God, God would not stand for blasphemy or saying what he did not say, or laying a claim, to something or someone, that God Himself did not appoint.
Christians do have the fear of the Lord, and do know that lying on God and the Holy Spirit, is unacceptable to God, who has all the Power.
Not because people do not understand the Holy Trinity, or Jesus, means that this is a lie or God did not work in these tenets of Christian faith and doctrine.
This is a simple solution, just ask God through Jesus Christ, to reveal Himself, as in the times past, and we have verification from the Holy Spirit, of the Truth of the Gospel of Jesus Christ.
Faith in God and Jesus supercedes any doubts in Paul, who God appointed, but also was a man of God, before coming to faith in Jesus Christ.
People also need to remember, that Record Keeping in Heaven is impeccable, and at the day of Judgment, the Books of Record will be opened.
God is not going to rely on silly mankind, or their biases, to bring back to memory, all the deeds done in the flesh while on earth, Satan's record too, and what the punishments are going to be, or rewards.
The bible says "Let God be True, and All men be liars."
The Records of Angels, saints, men, women, demons, kings, queens, princes, president, will all be Overseered through the Witness of the Most Holy Trinity.
"In the mouth of two or three Witnesses shall a thing be established."
Records are records, witnesses are witnesses, and God the Father, Son and Holy Spirit, work together, in the Unity of their One Purpose, or Sovereignty.
Whether people believe or not, is irrelevant to the Truth of God's word, both OT and NT
Paul's credentials as a Pharisee is impeccable, so his transformation should be an example.