This doctoral thesis studies Augustine's teaching on God or his theology as exposed in his four writings against so-called semi-pelagians ie. monks from Hadrumentum and from Suthern Gaul, and makes comparisons of these two doctrines. The work aims at the synthetic and meaningful presentation and analysis of the monks' and Augustine's texts and views, and at the obtaining of the complete insights into their doctrine on God and on the particular theological themes developed throughout their discussions. By analyzing Augustine's understanding of God and comparing it with his opponents’ opinions, the author wants to enlighten their starting points, the theological and anthropological consequences of their theological and anthropological premises, and to comprehend the reasons of Augustine's controversial theological positions. The work is divided into five chapters. The first chapter analyses the historical course and context of pelagian and so-called semi-pelagian discussion. The second chapter brings fort the Augustine’s teaching on God the Father and compares it with the teaching of so-called semipelagians. In the same way, the third chapter presents the teaching on God the Son, while the fourth chapter speaks about the Holy Spirit. The fifth chapter discusses different disputed theological topics of Augustine’s four writings, especially the theme of God’s omnipotence and invincibility of his will, and the themes of predestination, election and reprobation. The conclusion summarizes and presents the research results, offering the final synthesis and assessment. Concerning the method of the work, the historical-critical method is used while talking about historical circumstances of the pelagian and so-called semi-pelagian discussion, the systematic and synthetic method while presenting particular theological themes, and the analytic method as the way to evaluate and to review the stated propositions. The work has come to the conclusion that Augustine's understanding of God is to a large degree correct and accepted by the Church. His doctrine on gratuitousness of grace and on the beginning of faith is essential for the refuting of pelagianism. The agencies of Jesus Christ and of the Holy Spirit are viewed in the new light. Augustine sees the grace of Christ as the only force capable of bringing man back to the freedom, restoring man's capacity of good and meritorious deeds, and leading him to salvation. His innovative discourse on predestination, developed on the basis of his specific understanding of God's omnipotence, foreknowledge, invincibility of God's will, gratuitousness of grace, the original sin and of man's subjection to evil, is correct in certain positive postulates, but in certain implicit outcomes and consequences it appears to be defficient. The reasons are the additional negative repercussions on the situation of the reprobates, as well as the limits it brings to the agencies of the Son and the Holy Spirit. Although Augustine is not focused on the reprobates but on the elect, these controversial moments largely effect the image of God he depicts burdening it with the disturbing implications which cause it impossible to accept all the aspects of his understanding of predestination and of his doctrine on God.