Gospel Guidebook: Getting and Keeping It Right 
Modalistic Monarchianism Best Describes God's Nature
I am a strict monotheist who believes that God (i.e., Jehovah) is numerically one in person, possesses aseity, being self-existent, self-attesting (serving as His own criterion for truth), and self-justifying. Since the incarnation, God has revealed Himself distinctly as the Father (God as God) and Jesus Christ, the Son of God, (God as man), and has revealed His power and nearness more manifestly since Christ's ascension through the Holy Spirit (i.e., the Spirit of Christ). God has chosen to reveal Himself in these three modes of being in order to accomplish His eternal purpose in Christ, all for His own glory and name's sake. This belief has been historically called Modalistic Monarchianism.
The vast majority of Christians describe themselves as Trinitarian, but the truth is that many Trinitarians "are practical modalists" themselves (according to Trinitarian scholar Robert Letham on p. xxxii of The Holy Trinity). Modalism has a rich history in the church. In fact, the early Christian theologian Tertullian (155 to 220 AD) stated that modalists "always constitute the majority of believers" (Ch. III here), and this statement holds true today (see the video clips of Trinitarian apologist James White below). Tertullian was the first person to use the word "Trinity," but according to Letham, his teachings carried "a bias toward subordination and modalism" (The Holy Trinity, p. 100). In fact, scholars have long recognized that Nicene Trinitarianism gravitates toward modalism (Ibid., p. 290). Again, according to Letham, it was the Modalistic Monarchians who were happiest with the Creed of Nicaea 325 because it defined the Son as being of the same hypostasis as the Father (Ibid., p. 119). The truth is that the Trinity dogma wasn't really established until the Council of Constantinople in 381 AD. For example, the Encyclopedia Britannica says, "It was not until later in the 4th century that the distinctness of the three and their unity were brought together in a single orthodox doctrine of one essence and three persons" (here) and this statement is also verified by the New Catholic Encyclopedia (see p. 193 here).
More recently, Southern Baptist theologian Frank Stagg, speaking on the development of Trinity doctrine, said, "But what began as insistence upon tri-unity eventually became an emphasis upon the threeness and increasing jeopardy to the belief in oneness... To the term trinity were soon added the terms 'persons,' 'three persons,' 'three persons of the Godhead'...Thus trinitarianism was fast on the way to tritheism... This the New Testament never anticipated and does not support" (see p. 7 here). Stagg's concerns were correct because there are many Trinitarian apologists who are teaching that God ontologically has three centers of consciousness, which under any other name would be tritheism (see, for example, William Lane Craig here, James White here, Lane Tipton from 20:20 to 25:03 here, and the GotQuestions.org website here). In addition to Stagg, Swiss Reformed theologian Karl Barth emphasized that God is unipersonal and exists in "three modes of being" (p. 359 of Church Dogmatics Vol. 1). Likewise, Roman Catholic theologian Karl Rahner insisted that there is only one subject in God and famously said, "should the doctrine of the Trinity have to be dropped as false, the major part of religious literature could well remain virtually unchanged" (p. 10-11 of The Trinity), indicating that the Trinity doctrine held little significance for the bulk of Christian authors. For confirmation that Barth and Rahner held modalistic beliefs, see p. 197 of the New Catholic Encyclopedia here.
As mentioned above, James White has repeatedly said that 60 to 80% of confessing Trinitarians actually hold modalistic beliefs (here, here, here, and here). In other words, despite their nominal belief in the Trinity, they instinctively understand and read their Bibles with the comprehension that God is unipersonal and reveals Himself modalistically. The truth is that there is no text in the whole Bible that teaches that God is a Trinity consisting of three co-equal co-eternal god persons. Some Trinitarians try to appeal to 1 John 5:7 as a proof text for the Trinity, but it teaches no such thing. For example, textual critic Edward Hills has argued that 1 John 5:7 is authentic but was illicitly removed from early manuscripts because it was perceived to teach modalism, namely, that "the Father, the Son and the Holy Spirit were identical" (p. 212 here).
The Bible consistently teaches that God ontologically exists as one person. To grasp the truth of Modalistic Monarchianism for yourself, I recommend reading the article linked below. Personally, I became persuaded of Modalistic Monarchianism while reflecting on Luke 1:32-35 (especially the use of the future verb tenses and the conjunctive adverb "therefore") in consideration of passages such as Isaiah 42:8 and Isaiah 43:10-11. Also, I noticed how Romans 5:5-11 is in the context of self-sacrifice and how the Apostle Paul tells us that God the Father was emphatically demonstrating "His own love" in Christ as the Son of God. As a Modalistic Monarchian, I proclaim the self-sacrificing love of God, as opposed to one god person sacrificing another god person (Trinitarianism) or God performing human sacrifice (Unitarianism). To understand how a Modalistic Monarchian understanding of the nature of God distinguishes between the Father and Son, I recommend the following article by Oneness apologist Jason Dulle: The Acknowledgement and Proper Placement of the Distinction Between Father and Son.