If we (truly) believe, completeness (perfectness) will follow with perseverance of the saints.
1 truly believe - yes
2 "complete perfection" - we will be made perfect when we are with the Lord, until that time life is a struggle between doing the right thing or not. Remember the apostle Paul at the end of his life, wrote from Rome, that he did the things that he should not and did not do the things that he should...calling himself a wretched man. We are all wretched men/women.
Romans 7
15 For I do not understand what I am doing; for I am not practicing what I want
to do, but I do the very thing I hate.
16 However, if I do the very thing I do not want
to do, I agree with the Law, that
the Law is good.
17 But now, no longer am I
the one doing it, but sin that dwells in me.
18 For I know that good does not dwell in me, that is, in my flesh; for the willing is present in me, but the doing of the good
is not.
19 For the good that I want, I do not do, but I practice the very evil that I do not want.
20 But if I do the very thing I do not want, I am no longer
the one doing it, but sin that dwells in me.
21 I find then the principle that evil is present in me, the one who wants to do good.
22 For I joyfully agree with the law of God in the inner person,
23 but I see a different law in the parts of my body waging war against the law of my mind, and making me a prisoner of the law of sin,
the law which is in my body’s parts.
24 Wretched man that I am! Who will set me free from the body of this death?
25 Thanks be to God through Jesus Christ our Lord! So then, on the one hand I myself with my mind am serving the law of God, but on the other, with my flesh the law of sin.