According to Acts 1:15-26, the replacement of Judas Iscariot was necessary to fulfil the Scriptures. The disciples believed that it was in line with the prophetic writings of the Old Testament, particularly Psalms 69:25 and 109:8, which were quoted in Acts 1:20 (RSV):

20 For it is written in the book of Psalms,

‘Let his habitation become desolate,
and let there be no one to live in it’;

and

‘His office let another take.’

These passages were interpreted as foretelling the need for another to replace Judas among the apostles.