Acts 16:3

Him would Paul have to go forth with him; and he took and circumcised him because of the Jews that were in those parts: for they all knew that his father was a Greek.

To go forth with him ... Paul, seeing in this promising young man the qualities which would commend him to the work of a missionary, decided to take him along on the journey.

Circumcised him ... This was not for the purpose of enabling Timothy to become a Christian, for that he already was, having obeyed the gospel on the first tour. Neither was it for the purpose of admitting Timothy into any higher fellowship, or any more abundant grace; the reason for it being simply the one bluntly stated: "because of the Jews that were there."

For they all knew his father was a Greek ... This is to explain the anticipated objections from Jews. Knowing Timothy's father was a Greek, they would have assumed that Timothy had never been circumcised. Furthermore, they would have raised trouble for Paul over that issue wherever possible; and therefore, purely as a matter of expediency, Paul met it by the circumcision of Timothy. Those who have accused Paul of inconsistency in this, in the light of his adamant refusal to circumcise Titus (Galatians 2:3), have failed to discern the essential differences in the two situations. Titus, a Greek (thought by some to be Luke's brother), had no Jewish connection whatever; and there could have been no excuse at all for circumcising him, except, as the Pharisee Christians demanded, that of making his circumcision a precondition of salvation; and that Paul never for a moment allowed.