Justin Bieber encouraged his millions of Instagram followers to do a soul-check this Christmas as he plugged a book written by his best friend Pastor Judah Smith.

The dedicated Christian regularly shares messages about God and faith with his 103m followers and this time, used Instagram Stories on Sunday to plug Smith's 2016 book, 'How's Your Soul?'.

The book encourages readers to look beyond the externals of life to the one thing that really matters - their spiritual wellbeing and relationship with God.

The 'Where Are You Now?' singer, who recently tied the knot with model Hailey Baldwin, shared a snap of the book in his Instagram Stories, giving a subtle clue as to what's on his mind this festive season.

Bieber and Smith are such good friends that the pop star's Instagram profile picture is a photo of the pair together.

The love is clearly mutual as Smith has previously heaped praise on his long-time friend.

'I feel like I have learned more from Justin Bieber than he could ever have learned from me. I have been allowed to love him and I feel like we are family. We talk almost every day,' he said.

Bieber has been open about his Christian faith in interviews and Instagram posts, telling fans at Easter that the occasion had nothing to do with a bunny.

At Thanksgiving, he said in a heartfelt message that he wanted to be more like Jesus.

'Happy late thanksgiving to everyone. First thanksgiving as a married man, first time hosting thanksgiving. First time both sides of the family has come together,' he said.

'Relationships are hard and love isn't always easy but thank you Jesus for showing me how! Every day is a learning process, trying to be more like him, patient, kind, selfless, boy I got a long way! But gods grace is sufficient!'