Only one Lord created this entire universe. He should have told the same knowledge everywhere in the world. The contradiction is only due to misunderstanding. In Hinduism also Sankara says that again human birth is almost impossible (Jantunaam Nara Janma Durlabhamidam). Sankara was the incarnation of Siva and so what He told is also authoritative. Even Gita says that the soul comes back to this world but does not say that the soul gets again the human birth (Ksheene Punye Martyalokam Visanti). In the second chapter, Gita speaks about rebirth but not about the human rebirth. Islam and Christianity say that the human birth is given only as a single chance. After this the final judgement is given. The soul either goes to the Lord or goes to the hell permanently. In Gita also there are two ways for the soul. Either the soul goes to the Lord and does not return back or the soul returns back to the world (Abrahma Bhuvanath, Yat Gatva). According to Gita, if the soul does not go to the Lord (Brahmaloka), it returns back after enjoying the fruits of incomplete spiritual effort. Therefore in this human birth, if the spiritual effort is completed, the soul goes to the Brahmaloka permanently.
If the spiritual effort is incomplete the soul may go up to any world below Brahmaloka, it will return back to this earth after enjoying the fruits of its incomplete spiritual effort. The soul may go up to the sixth world, it cannot go to the Brahmaloka, which is the seventh world by doing the remaining spiritual effort in the sixth world. The reason is once the soul leaves this earth all the upper worlds are only Bhogalokas i.e., the worlds in which the soul can enjoy the fruits, but cannot do any work (Karma). Therefore the soul has to return back to the earth. The soul will not get again another chance of human birth because once it is failed it can never succeed. Therefore the soul comes to the earth and falls into the cycle of animals and birds only. When the soul is trapped in this cycle of animal births, it is treated as a permanent hell.
If the soul goes to Brahmaloka it always accompanies the Lord either in Brahmaloka or may come back to the earth along with the Lord who takes the human incarnation. In such case the soul is born as a divine servant of the Lord. Such soul will not take rebirth in the cycle of animals. Thus for the divine soul also there is no such rebirth. In this way Hinduism, Islam and Christianity are correlated as one concept preached by the one Lord. Christians and Muslims are putting up sincere spiritual effort because there is a threat that this human life is the only chance. There is no reexamination according to these religions. This human birth is the only one examination and the soul either passes or fails and will not be allowed for any reexamination. But in Hinduism such threat is not there.
People have taken a lenient view on the spiritual life because they think that the human rebirths are possible and so they can put up the spiritual effort slowly in the future human births. The Lord cannot say different theories to different people. The rule must be same for all the human beings of the world. Therefore whatever the Lord told in Christianity and Islam also told the same in Hinduism. Hindus misunderstood the concept. Therefore what ever may be the religion, every human being should think “Now or Never”.
The human rebirth is only for Yoga Bhrashta i.e., the soul, which has reached Brahmaloka and fell due to some slip. Such a soul is suspended from Brahmaloka and comes down to the earth and takes rebirth as a human being only. You have passed the P.G. Degree and obtained the post of the lecturer. But you were suspended for a month due to some mistake. You will be re-appointed.
Similarly the Yoga Bhrashta will be born as a human being for some time and will come back to Brahmaloka. The case of incomplete spiritual effort is different from Yoga Bhrashta. The incomplete spiritual person is like a B.A. Degree holder who never achieved the lecturer post. You cannot argue that you can be appointed as a seventy five percent lecturer since you reached seventy five percent of the total educational period (from school to P.G.Degree is hundred percent) by getting a B.A. Degree.
Thus there is no partial achievement by partial spiritual effort. The grace of the Lord is either hundred percent or zero. If it is hundred percent you are reaching Brahmaloka. If it is Zero percent you are falling back to the earth in the cycle of animals and birds. There is no third way in between these two. When this truth is revealed, only one in thousands will try to put the real spiritual effort as said in Gita (Manushyaanaam Sahasreshu). By this Hindus will become alert in spiritual effort like Christians and Muslims.