Transformation Process

Romans 12:2 (AMP)
2 And do not be conformed to this world [any longer with its superficial values and customs], but be [a]transformed and progressively changed [as you mature spiritually] by the renewing of your mind [focusing on godly values and ethical attitudes], so that you may prove [for yourselves] what the will of God is, that which is good and acceptable and perfect [in His plan and purpose for you].

Transformation is a process, it is not an event. As soon as we surrender our life to Jesus, then we set off on a journey of transformation that God takes us through. You and I cannot transform ourselves thus we have to willingly allow God to transform us as we follow the lead of the Holy Spirit in our lives. True transformation cannot take place until there is surrender. You and I have to trust God enough to allow him to lead and direct us according to his plan and purpose.
When the Holy Spirit begins to work in us then we are introduced to a new way of life, a new way of thinking, a new way to do life. This sometimes becomes a struggle within us because remember we are still thinking like the old man which is why Paul tells us that we are transformed progressively as our mind is renewed. Paul himself faced the same struggle when he said;

Romans 7:15
15 For I do not understand my own actions [I am baffled and bewildered by them]. I do not practice what I want to do, but I am doing the very thing I hate [and yielding to my human nature, my worldliness—my sinful capacity].

What you and I have to understand is that after we have embraced Christ as Lord and savior, then God begins to take us through a process of transformation. If we remain in Christ then we begin to show forth fruit and transformation. As we read his word then our way of thinking is challenged and renewed and the way we live life continues to go through a transformation. That is why it is important to stay in the word because as we do so then God through the Holy Spirit continues to give us greater insight with leads to renewal of our mind and understanding to embrace and walk in the freedom and victory God has made available for us.
Remember Jesus said

John 15:4-5
4 Remain in Me, and I [will remain] in you. Just as no branch can bear fruit by itself without remaining in the vine, neither can you [bear fruit, producing evidence of your faith] unless you