The Bible shows very clearly that there is only one God, and yet that there are three personal distinctions in His complex nature, traditionally referred to as "three Persons in the Godhead" God the Father, God the Son, and God the Holy Spirit.
Each is distinct from the others but never acts independently. They are one in nature and purpose. This mystery is called the doctrine of the Trinity, though that term is not used in the Bible. The teaching, however, is present in seed form in the Old Testament and is revealed explicitly in the New Testament. Note passages such as Matthew 28:19; John 10:30, 14:26; 2 Corinthians 13:14.
Our finite minds cannot understand or explain this mystery of God, which is nevertheless a fact. We must accept the truths found in the Word of God by faith even though we ourselves cannot comprehend them fully; read Hebrews 11:1, 3, 6 and 1 Corinthians 2:5-10, 14; 13:12.
It is really not surprising that the infinite God should be complex in His nature beyond the ability of finite humans to comprehend! This doctrine is absolutely essential to New Testament Christianity. Theologians have pointed out that if it were not true, the Bible would be unreliable, Christ would not be divine, and His death on the cross would not atone for our sins, being merely the death of a martyr.