Summary: Nothing is too hard for God!
My understanding is that Islamics and even some groups who use the term Christian have a problem with Jesus being the Son of God. After all, one means one how can God have a Son? While I understand that the doctrine of the Trinity or Tri-unity of God is hard for the human mind to grasp, it is clearly taught in Scripture.
26 And God said, Let us make man in our image, after our likeness: and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth.
27 So God created man in his own image, in the image of God created he him; male and female created he them. KJV
Right here in the first chapter of Scripture we have a problem posed for modalists and other non-trinitarians. If God is not a plural unity then who is He speaking to when He says Our image. Certainly not the angels for He would not say Our when talking to them for they are created being as well. It could only be to someone of equal stature as a creator and He is the only Creator. Twelve times in Scripture it is said that He is the Lord God and there is none else. So who comprises the Our in Our image. To have an Our you have to have at least two so we have a Deity that is at least a Bi-Unity in this passage though we shall see in other places the Tri-Unity.
12 Hearken unto me, O Jacob and Israel, my called; I am he; I am the first, I also am the last.
13 Mine hand also hath laid the foundation of the earth, and my right hand hath spanned the heavens: when I call unto them, they stand up together.
14 All ye, assemble yourselves, and hear; which among them hath declared these things? The LORD hath loved him: he will do his pleasure on Babylon, and his arm shall be on the Chaldeans.
15 I, even I, have spoken; yea, I have called him: I have brought him, and he shall make his way prosperous.
16 Come ye near unto me, hear ye this; I have not spoken in secret from the beginning; from the time that it was, there am I: and now the Lord GOD, and his Spirit, hath sent me.
17 Thus saith the LORD, thy Redeemer, the Holy One of Israel; I am the LORD thy God which teacheth thee to profit, which leadeth thee by the way that thou shouldest go. KJV
In this passage, it is clearly God speaking to His people and may I say that he is not happy with them. Verse 12 affirms the one and only God as the first and the last or as the New Testament puts it the Alpha and Omega. He says, it’s me and me alone. Yet, in verse 16 we see a presentation of the Holy One of Israel that seems different than what the Sh’ma presents in Duet 6:4 which is a foundation stone in Hebrew theology. “Hear, O Israel: The LORD our God is one LORD:” KJV
So God is speaking, the one and none else God, but He says that the Lord GOD and His Spirit has sent Him. Hello, that is a three count. The speaker calls Himself God. He said God sent Him. He also said God’s Spirit sent Him. Some say that the Spirit is just an influence so we still have a Bi-Unity if you take that position. Yet, an influence is not a sending on commissioning agent. That requires a persona or person. Thus two Persons of Deity level sent this Deity that is speaking hence three persons or a Tri-Unity/Trinity.