For those who do not know it, John's Gospel was written by Jesus's disciple, who was the most spiritually advanced of the disciples, and the only one who remained loyal to Jesus after his arrest and who was present at the crucifixion - where he was given the care of Mary, the Mother of Christ.
John's Gospel it is the only direct, detailed eye witness account of the life of Jesus - although such accounts are embedded elsewhere the New Testament. (Furthermore, the disciple John is - so the end of this Gospel tells us - still alive and at work.)
Among all of scripture, John's Gospel therefore has immense and distinctive authority.
The take-home message of this Gospel is perhaps the most wonder-full, positive and optimistic statement of the human condition that has ever been. Such is the simplicity, that to attempt summary leads to an impression of platitudinous bathos.
But importantly we are instructed to be of good cheer and not to worry, to be loving - especially among Christians, to be engaged in the world, but indifferent to the world's opinion.
In sum, John's Gospel teaches us what it should be like to 'believe on' Christ; and if our lives are not like this, then we are not living it. In particular, if we are filled with angst and fear of the future, and are focused on what 'the world' thinks and does - then we are (so far) missing the point.
We are also told firmly that - because of the example and teaching of Christ and the continued witness of the Holy Ghost - we know everything we need to know: we know the nature of God, and that we share in that nature; we know that both the Son and the Father regard us like beloved friends, and not as servants or vassals; we know what we should do and how we should be; and we know that there is ultimately nothing stopping us doing everything necessary now.