Knowing that our presumptions can influence our understanding, hopefully without doing injustice to God's word, let's consider alternative readings to a few of the "pro-Trinity" verses. There may be better explanations, but these alternative interpretations should give rise to serious thought.
Col. 1:15 can be seen as God’s intention in His plan to credit His Son even with creation, “For by [better: through] Him all things were created. If Christ were to be the perfect expression of God, in his role as Lord he could not be lacking in any aspect. He necessarily had to share in all power, honor and glory of the Father. Hard to grasp? Consider that God, in a similar way, has attributed to believers all the riches of Christ, so that one day we also will share in His glory.
Phil 2:5-8, so often used to support Trinitarian belief, is essentially an appeal by the apostle Paul for believers to follow after the humility exhibited by Christ. The mention of “being in the form of God” raises the question of what is meant by “form.” Since God who is invisible has no “form” the word must refer to the form (likeness?) of Christ, who is the image of God.
John 10:30 When the Jews accused Jesus of blasphemy by saying “I and my Father are one,” he referred them to Psalm 82:6. Rather than asserting he was equal with God, he compared himself with “the children of the most High,” sons of God, of whom He was foremost. See John 10:30-39.
John 17:5 In asking to be glorified with “the glory which I had with you before the world was,” Remember that Jesus knew all that was written of him in scripture. Then, it seems most reasonable to assume He was referring to the future glory destined to be his, foretold in the scriptures according to the plan of God.
John 20:28 When Thomas exclaimed to Jesus, “My Lord and my God!” was he affirming that Christ was the Father, the one true God, or was he praising Christ as the very image of God? In the Old Testament when God revealed himself through angels in the likeness of men, those to whom God appeared said the same thing, yet certainly the angels were not God, only a means through whom He could reveal Himself (cf. Genesis 32:3, Judges 13:22).